Of all things he had to start another WP hosting company? He has the name and the money to do something bigger. I can't think of any motivation other than making money that went into this.
That being said, the staging area for testing changes seem like a really useful feature that I haven't seen in any other WP hosting service.
Taking the largest, most vibrant blogging (and now general website and for some CMS) platform on Earth and trying to not only make it awesome but contribute new features and plugins back to the world isn't big enough for you?
Finally, you should know that for me "doing something bigger" isn't a motivation. I don't have a need to "change the world" or somesuch thing. I want to do something valuable, something understandable, something fun and interesting, and yes of course also make money.
I'm not sure if I'd call WP Engine whipping out your dick and getting a ruler, though.