The mainstream audience and the critics want different things from movies.
The critics & movie buffs like discussing movies afterwards. If a movie can somehow provoke that discussion through ideas or themes, then critics seem to enjoy it.
The audience simply wants an emotion reaction from a movie, and they want to be entertained. They don't discuss movies beyond saying "yeah that was a good movie". The audience wants to walk away feeling something. If that feeling is mindless rollercoast-like entertainment (like Transformers), then so be it.
Also, the mainstream doesn't think much of Transformers. I've never met a single adult who thought highly of Transformers. Transformers has been a popular money machine long before the movies.