I'll give it the diversity factor, they are diverse sides thought up by games workshop, but the balance being perfect? After eight years of patches yeah.

The interface was horrid, even for when it was made. With a couple dozen units on each side it was playable, with a hundred it was a nightmare, and I was fast back then. CNC at least didn't have the horrific limitations that made large scale battles such a goddamn pain in the ass, and using abilities in starcraft amounted to pretending you were playing a small scale tactics game while attempting to manage a large scale war. It was better than warcraft though, which was even worse than dune.
My theory on why blizzard does so well is that too many people drool over cutscenes. Their games have universally had poor graphics(go through the list, look at the dates, and compare them even just inside genres, they're all two year old crap graphics that someone else already did, from warcraft to WOW), mediocre gameplay with the possible and arguable exception of starcraft the clunky clickfest, and been there done that stories just like everyone else, but with better cutscenes and voice actors, an advantage over westwood that they lost with RA2. You can't beat softcore porn stars playing special forces apparently.