For one by giving players the ability to reach places faster (with mounts) you completely kill (or actually revive) ended battles. How? We all know that you can help downed players, everyone can revive killed players. By making the player able to reach the battle over and over again really fast, the battles would never cease. Would just keep happening and happening. And this isn't just my point of view, it was also pointed from one of the Anet designers. For this reason alone mounts should really be kept away from WvW. (In fact I just agree for it to be in PvE and by only being cosmetic).
2nd is that there is a lot of strategy to actually buy a waypoint in a map. Right now it's not quite common just because everyone is still working their money, building their trusted ranks of the guild and more. But in fact by having one hugely improves the "tide" of a battle, and mounts would really disrupt that.
And 3rd: When I die in battle and rez I will often think "I need to rejoin that battle" and walk towards it but in the middle of it I find an event needing help, or I find some wanderer going on (easy prey ) or some minor battles and etc. To be quite honest with you thats very good! You can't just rush thru stuff, IF you really want to rejoin the battle you died you probably need to go around other battles, sometimes even Kite enemies and etc, but you can also just forget about that battle and just help elsewhere.
Yes, this. Furthermore each proffesion has access to skills and traits that grant Swiftness and I'm not event talking about dirt cheat foodbuffs. I also thought that no mounts is a bad thing, especially in WvW, but after playing more and getting better I see that it was well thought decision.