The only problem with direct injection and the aftermarket would be......well think about what it would cost for a set of 6 1000cc injectors for a direct injection motor.
ECU's that could run said direct injection.
Notice the Track oriantated Porsche 911's (GT2 and GT3) still run conventional injectors, so that aftermarket ECU's can run the things.
Sure technology will catch up, but at what price, and with a somewhat limited use atm.