The difference being that Serious Sam wasn't a clone of any other game on the market at the time. It was a breath of fresh air, a return to the classic days of shooters like Doom. When you played Serious Sam, you didn't think "I've played something very much like this before, and the other game was better".
"Gore", on the other hand, is a total rehash of Unreal Tournament/Quake 3/whatever. And it's not, actually, very much fun. The game balance is nonexistant, the weapons are pathetic, the physics are abysmal... I went back and reinstalled Unreal Tournament, just to check, and it's infinitely more playable in every way. And Unreal Tournament sucks compared to Quake 3.
The Gore Guys should just admit that they wanted to make their own Quake game, just like so many other developers wanted to do. (Anybody else remember "Chasm: The Rift"?)