according to the rumor, they tried to do this, but PD wouldn't allow it.
Kaz knows more about brand dilution than most it seems.
Makes total sense to try and start a new IP at the start of a generation, it's the time when people are most receptive and two strong brands are better than one for long term viability.
Besides if they are aiming for a different type of game (which I believe they are) which is more 'fantasy/dream car' focused as opposed to GT's (alleged) focus on real motorsport then it would make little sense. If MS released 'Forza: PGR' people would rightly be confused.