I would not outright call the PlayStation 3 superior hardware, it depends on how you look at it.
To break it down easily for the non-nerds among us, there are just things some consoles can do better than others, and some things one can do, but the other cannot. The Xbox 360 is better with textures, the PlayStation 3 is better with shadows, etc etc. It is the reason the gore was severely toned down on Ninja Gaiden Sigma 2, the PlayStation 3 just could not handle that many textures.
Most likely Platinum wanted Bayonetta to be an Xbox 360 exclusive, but SEGA was like "Hey, I know how we can make more money!". I am betting on the engine just being scrapped honestly, might as well just make something that works great on both platforms and can cut down production time. If they could, I am sure they would put it on as many platforms as they could within budget.
But I guess SEGA is insane at times and do not really care so they might just do anything.