^^^ What they said. Electric turbos just don't build enough boost to be terribly useful. That being said, Audi is actually playing with electric turbos on their diesel cars at the moment. They are using the electric turbos to build boost at low RPMs, until the revs build enough that the larger mechanical turbo starts to kick in, providing top end boost. It's quite a nifty way to reduce turbo lag.
Audi’s Secret Super Diesel Electric Turbo Prototype Is Fast As Hell