Because they make it open world and then it sells more even though the overall design suffers

Because what other superheroes have such a ready made setting for them?

>Arkham Asylum is a large facility that's also very old so not only is there the "modern" facility there's god knows how much of the old buildings around and beneath it
>it's safely set away from the city proper so you don't need to justify why you can't leave the boundaries
>most of Batman's villains are mental patients who get incarcerated there so it's no stretch to say most of his rogue's gallery is present when the game starts
>the setup of Wayne Enterprises heavily investing in it, as well as it being so close to the city, easily justifies the power up system being slowly unlocked over time, naturally allowing for an increase in abilities that don't feel forced
And, of course
>Batman is still just human so all of his opponents are a threat to him, especially so many over such a period of time, so there's no incongruity in why he's having such an easy/hard time dispatching either the mooks or the bosses

It was basically the perfect storm of decent videogame design. No other hero in DC or Marvel has anything that comes close.