That's the easy and somewhat circular answer. I'd be curious to know what rationale the service had for not integrating with Google Maps or vice versa. Cost is the only legitimate one that comes to mind - I can't imagine concerns about open data were a massive issue given that third parties have been using and providing a subset of the same data for some time.
Google didn't want more transit partners for a long time: the sign-up page just said "check back later".
I think they are very concerned that the data feed will be bad or unreliable, and that they will be providing bad directions. I know in NYC, the schedule and such for subways is generally simple, but has crazy exception rules every weekend for maintenance.