because pre-cata era that was how alliance was set ? prior to varian return alliance was on serious boiling point, alliance even only faction i can name that had change of leader (from Bolvar to Benedicus then back) during vanilla to show how much power struggle they have
alliance was shown as faction that is far stronger than horde, but have zero unity, nelfs look high on anyone else, gnomes f8cked themselves up, dwarfs hate each other the most than anyone else (playable dwarf is only alliance faction that can be at war with wildhammer dwarf faction), and human noble power struggle was VERY obvious since even lvl 10 quests
It made alliance far more interesting than this boring whatever we have now
Then blizz turned the table on everyone head and made alliance horde and vice versa, i remember ppl even in-game calling alliance blue horde and red alliance during cata when horde started to have racist tension and tear itself apart like how alliance were in early days (but more extreme), now when u talk about a faction that is full of racists, races that don't tolerate each others and sub-factions inside and constant power shift, it is the horde...
So if u follow warcraft story since wc3, it used to be alliance that had internal war, while horde united, not other way around
Now to add insult to injury, blizz show alliance losing (how?) against mainly Sylvanas & co horde, a sub-faction of horde, how no idea