Namco have said that they wanted everyone to experience Tekken 6, which is why it was made available on the 360 as well as the PS3.

“We want everyone to experience it. This is the first time that Tekken’s moved away from the PlayStation format. It’s no longer exclusive to Sony, so it is on Xbox 360… The install base for both machines is pretty huge, and we have to make it available to everybody now.

We would guess it involves the price it costs to develop games lately, making this comment a simple 'we did it to make more money'. It's understandable, I sure as hell couldn't afford to make a game. Not that I would. Or am a company.