lol, is it your first Blizzard franchise or something? Blizzard doesn't crank out games like they're CoD. They generally do have a fairly long development time, but they certainly haven't been actually working on Diablo III since the release of Diablo 2. They tend to be of the mindset that releasing a mind-blowingly great game late is better than releasing a pretty good game on time.
There was no rush because Diablo 2 is *still* a good, playable game. There are many, many people who still play D2, and as old as it is, it has stood the test of time far better than most games.
It was 12 years between the release of Starcraft 1 and Starcraft 2, and 7 years between Warcraft 2 and Warcraft 3. They don't make a game then hop directly into making the sequel. They release one when it's been long enough to really feel like a new game in the series adds value. Just like how Starcraft 1 was still doing fine when SC2 game out, so is Diablo 2.
Anyway, you seem to be somewhat of an entitled brat.
there are much better RTS out there.
Than SC2? Not really. Arguably, SC:BW is a better RTS, but that's about it.