Bias, nope.
But do they forget about the alliance sometimes. Or focus more on the horde....yes.
Main problems people in this thread have are: story, allied races, racials, zones, focus, hero's etc.
People say: But horde towns are attacked more. Yes , but you always win it back. We just got 1 zone destroyed, 1 zone turned into a camp for kill night elves and 1 city turned into ash. And alliance city's are attacked to. But less yes. So less focus on them.
Horde leaders: Yes they are replaced fast, but they have big epic stories around them. most alliance hero's have not. Our last leader had some stories but died. His son is pretty much there to help along a horde hero's story. Always on the sideline.
Allied races: while horde allied races have their problems. They are what they wanted ( z trolls, other orcs etc). Alliance did not get what we wanted ( wildhammer, high elves, highborne, vry kul). Hell void elves and lightfored and mecha gnomes are not even races. There are void creaturs, the leader of the lightforged is a human and mecha gnomes are just normal gnomes with mad max stuff. SO you could not even count them as "races" .
Racials: Yes pvp racials where stronger in the start for the alliance. But for years horde racials have been supreme.
Zones/city: yes here the alliance wins ( for once).
But lets look at expansions why we think its more horde focussed:
tbc was good for both races.
Wotlk: alliance
Cata: we helped green jezus out the whole time.
MoP: started out as equal. But quickly became a horde civil war stuff and raid.
WoD: again, the start was equal. But they cut the alliance focussed raids. Story stop real fast. All it was horde , horde....and horde.
Legion: high mountain and val shara cancel eachother out. But suramar was a horde focussed zone, and patch all about a new allied race. Yes we got argus. with the very few quests there.
Bfa: Zone content Alliance wins, story overall again horde wins. It was about Saurfang, sillybowlady etc etc. Hell even the big city raid the alliance could be replaced with any other faction and it would not change the story that much.
So in my eyes/opinion. Its more horde focussed.
Think racials, allied races and story focus are the most to blame.