Why?

For one thing, the curved keyboard makes it easier for small hands to stretch octaves (since the endpoints are closer together). It also looks cool, and makes you feel important to be in the middle of it (like a DJ or organist).

It would be fun and virtuostic to play treble parts with the right hand and bass parts with the left hand without looking at either.

The pianist would constantly be spinning around on his stool to look at different parts of the keyboard. This means that, in concert halls, there are no lousy seats where the audience has to look at the pianist's back. Everyone in the audience will see the pianist some of the time.

In fact, a whole system of circular concert halls could be built to accommodate circular pianos, harpsichords, and organs.

The fancier pianos would have gaps in the frame every 60 degrees or so (where the frame turns at an angle to correspond with the more sharply curved keyboard). These gaps can be used by the pianist to store additional percussion instruments, coffee, magazines, or other concert supplies.