Why weren't there more successful Blizzard game clones?

When games like Half life came out and became so successful and profitable, many companies cloned it in one way or another. Now there are a huge variety of FPS games.

Why hasn't a lot of game companies been able to clone a game like Diablo, especially since it would probably make so much money if done properly. Titan's Quest is probably the only one that comes close. Most have failed. What was it about Diablo that made it so great, and why hasn't any other companies been able to make a game that surpasses it? It has been so long since Diablo came out.

When games like Half life came out and became so successful and profitable, many companies cloned it in one way or another. Now there are a huge variety of FPS games.

Why hasn't a lot of game companies been able to clone a game like Diablo, especially since it would probably make so much money if done properly. Titan's Quest is probably the only one that comes close. Most have failed. What was it about Diablo that made it so great, and why hasn't any other companies been able to make a game that surpasses it? It has been so long since Diablo came out.

I can tell you the number one reason I didn't play Titan Quest; they didn't have a system in place similar to "closed" battle.net with characters that can't be hacked. If it had that, I definitely would have played a ton of that game. (Yes, I know about the duping bug in Diablo II, but that was a fluke hole in the game, and not just simply editing the files on your machine. That's not the same thing.)

When your intent is to copy a succesful game franchise you have already given up on surpassing it.

I may be able to recreate the mona Lisa and make her more modern, more detailedand more attractive to my standards, but it would never be considered better than the original.

Go ahead and make clones and copies, but just by creating them you give the originals even more credibility and value.

Plain and simply, no one comes close to what Blizzard can do