They've grown fast and a lot after Unity 2.X
Before they were like other 3D engine companies just beginning, but already in the beginning when there was only Mac support lot of pople was dreaming of having such workflow on Windows, what comed to reality !
Another big point is the marketing they do is really good, like conferences, mail news on incoming new features nad updates etc ...

But i just think the thing that have boosted the engine popularity have simply been the Iphone support indeed. lot of pople jumped to Unity caus it was prooven, with lot of published games in the Store, and a
great workflow indeed.
Lot of pople have done some good money with it, some have jumped to Pro after some success on a game for the Store.

Not all indie engines have an ARMY of skilled PROGRAMMERS behind
And a strong advertising strategy indeed.

I think others engines have their own users also, specially when you see the price of the Pro version of Unity for iphone or Windows.
And not all people will spend such amount of money to have real time shadows and for making indie games !

Well i hope one day to see some A9 for Iphone !