1. Batman is usually the hottest DC franchise in mainstream, especially now with Arkham games and Nolan movies
2. Batman's rogue gallery is large and popular with lots of great personalities, many say it's the best from all comics
3. Batman is down-to-earth that fits a melee oriented fighting game perfectly
4. Even if there end up being 10 Batman characters, the game still has 20 other characters so calling it Batman would be wrong
It's all speculation and wishful thinking when Boon has said there will be popular and unpopular characters, but having Batman makes sense businesswise. Two-Face or Poison Ivy are more known than Doctor Manhattan or Hawkman. Especially when apparently many comic fans are already shooting down the game because "Nightwing can't beat Superman", so gamers and fighting gamers seem to be the main audience.
We know where Marvel Vs Capcom 3 got with it. Some margins love Rocket Raccoon or MODOK (I love MODOK), but forums are filled with "RR lulz, where is my Cyclops?!". A fighting game relying on popular characters should go for the most popular must-haves first if they have any moveset potential. For Batman I'd say Batman, Robin, Joker, Catwoman, Riddler, Penguin, Two-Face, Bane, Poison Ivy and Mr Freeze are characters like that - most anyone who haven't ever even read any comic can recognize them and they are a big pool for gamers to pick old favorites from as an entry point. You can even take out a couple from that list.
But I can't really get into gameplay. If Hawkman truly plays better than Riddler then by all means add Hawkman playable instead. Just saying that conceptwise more popular ones aka Batman ones make more sense and it's no wonder many are asking for Batman characters.
Anyway 99.9% certainly the roster is already set in stone so wishes will affect DLC at best.