Multiple reasons: The game system since they think it's too luck based even though it isnt, DOA gives you a chance to counter on read but outsiders often think it's just random. The fanservice also tends to put off people from playing too
The fact that most fans and the publishers/devs sell the game for the fanservice more than for its gameplay is probably the main reason... I would never get into DOA if it wasn't for Ninja Gaiden opening me up, because from outside, it really looks like a game that people play for the boobs only.
The FGC doesn't respect DOA because its own community doesn't do enough to give them a reason to respect it. I.e. Look at the Smash community. shitloads of people come out to their events and thus they have made an impact at the biggest stages such as EVO. Whereas the DOA community is too busy bitching about online spammers and what DLC costumes they want.