Also, I think I remember you having to sacrifice that airship in the blast because it wasn't fast enough to get away... Anyway, I would expect all alliance cities to be defended by stronger air defenses this time around since we know how far you are willing to go.
The Horde had a one-use weapon capable of destroying a small city. Jania can enslave a bunch of elementals and wipe out a city any time she pleases. Plus Dalaran. Plus indestructible hero airships. etc. etc. etc. The Horde only survives because they pay 50% of the subscription base, not because they logically should. The entire game is massively slanted in the Ally favor.
You can't apply logic to anything relating to the war in the game. It is entirely plot-based and changes all the rules based on what quest you're on at the time. Some quests have you take control of one vehicle or cannon or whatever and kill literally hundreds of guys.
The starting MLP quest where the alliance flying gunship rolls out above that area, deploys dozens of aircraft each killing dozens of horde soldiers each, then sinking several ships as well easily, etc. It just changes from quest to quest and theres no point really having any kind of more powerful navy or whatever for alliance when the horde are just going to get some random quest to sink 10 alliance ships with zero effort. It would be nice to have more realism in the game, but unless its a complete overhaul something like this wouldn't make any difference.
I like ponies and I really don't care what you have to say about that.
historically, while a success in the long run, (war) ships powered by steam engines had a few, potentially rather critical, drawbacks:
usually very underestimated but maybe actually the biggest problem was needing large coal depots all along any coast line the ship would travel. preferably ones that were run by the same country as the ship^^ on high sea travel, using the steam engine was literally impossible. which means that they'd still need sails for pretty much anything other than combat - which in turn leads to point two.
the ships would either have smaller/less sails or the weight of the engine and coal supplies added to whatever weight a 'normal' ship would have. and both these options result in quite significantly lower (traveling) speed.
combat speed would obviously be less affected, because a good (early) steam engine would be about as fast as an average ship of it's size with good wind. still, the maneuverability might be affected by the added weight regardless of what force is used to actually 'power' the ship
even with the, pretty much mandatory, metal armor plating protecting the engine, funnel and paddle wheels, a lucky shot (or several well placed ones) from an enemy cannon could easily result in an explosion that would destroy the entire ship in one moment.
granted, that was always possible, due to war ships having to carry large amounts of gunpowder for cannons, but actually destroying a ship by setting one of these aflame from afar was very, very rare. the steam ship has that same gunpowder, plus an engine that is not only much bigger but also doesn't even need to be set aflame to explode; a big hole or bent piece of metal in the wrong place could already be enough.
in the early stages of steam powered industry, those engines were anything but reliable. the risk of it 'breaking' or even exploding due to small mistakes in handling or production was pretty damn big, accidents common. which, on a side note, also didn't exactly help the morale of any men having to engineer one these dangerous things - and that's without beeing on a rocking ship at sea, with saltwater everywhere, enemies firing at you and last but not least using massive, heavily kicking cannons yourself.
those things were expensive. in production, 'fuel needed, maintenance.
the really big advantage steam powered ships had was beeing able to control the ship's speed regardless of wind strenght and direction. that advantage could be huge if executed with proper tactics, and a bit of luck.
still, it took quite a long time from the first steam powered war ships, to them becoming common.
as for the actual question at hand: i don't know, maybe blizzard likes big blue sails?^^
i don't actually think there's much reasoning behind alliance not using more of them, other than 'what looks cool'. especially since weight doesn't seem to be an issue. but if there is a reason (or if we are going to pretend there is one, for lore's sake) chances are it's one of the above.
Horde fleet destroyed by Alliance airship.
Horde base destroyed by Alliance landing party.
Horde airship destroyed by Alliance air force.
Horde survivors destroy Alliance air base.
Both sides gather local allys.
Local allys wiped out when the Sha busts out.
Both sides make small base in Kun Lai.
Both sides build bases in Krasang Wilds.
Horde assault force wide out to the last in the jungle. (A Little Patience)
Horde base nearly wiped out by Ally assault force. (Domination Point)
Nothing much on HvA front yet.
They aren't that fast.
They're huge lumbering beasts meant to punch through icy waters. Moreover, they lack maneuverability and weapons which makes them GIANT TARGETS.
Finally, construction and temporal costs. These probably take way more and way longer to build and still don't provide the same kind of offensive capabilities as the battle ships.
as for the magic, i think it's save to assume that if moving such big objects, at least in a reliable way, would be that simple, we would've seen it done many times already - again, assuming there is any actual reasoning behind this other than what looks and/or feels good in the game.
To what I was going to say:
---------- Post added 2013-04-07 at 08:52 PM ----------
Also I just figured I'd share this. The Alliance has about 200 ships. Each ship probably has about 15-20 people each. So the Alliance has roughly 4,000 navy members.
I assume it's just a cut-and-paste mirror image of Domination Point?
(Our base was almost destroyed, but we won thanks to the players who wont get any credit in the books or comics and will probably be replaced by a Lore Hero solo-defeating the entire enemy force just like Onyxia!)
so it might as well just be a warcraft-universe steam engine. the exhaust pipes are probably just not there because it didn't look as good.
well, 1) an entire city full of the greatest mages was needed to it and 2) slowly moving something from point A to point B is not suitable for ship combat. and even if it took only a few mages, and even if they could do it at a decent speed, and even if they could keep it up for a decent amount of time, it would still most likely lack the maneuverability that is very much needed for naval combat.To what I was going to say:
Alliance also still have a submarine from Cataclysm don't they? I think it was damaged but not destroyed by the kraken.
The Alliance had a large portion of its fleet wiped out by the Horde controlled Kraken just before MoP, though. Supposedly they're building up as quickly as they can, though.
---------- Post added 2013-04-07 at 09:25 PM ----------
Edit: Hit the reply button too soon and missed the entire point of what I was going to say. Will reply below.