Because stubborn people continued to play them through to completion, I suppose. Not that I'm saying you are stubborn, no, definitely not.
Bought all 3 off Gamers Gate a while ago. Played 1 & 2 way back in the past. I really liked 1 at the time, but 2 was hate at first sight. Small linear levels, like an effin corridor shooter. Still hope I find the time some day to play Guerilla.
Edit to add: It has to work hard to beat X-Com 3 and JA2 for sheer destructible environment fun.