Because it´s .... not bad at all ?
I think if we stick to any particular game for a long time, it´s because that game probably has something that feel good to us.
There are some games that I played extensively, but were considered "mediocre" by critics in general. A good example is Doctor Lautrec and the Forgotten Knights (3ds). I only heard and read bad things about that game, but when I got it, it was actually fun and engaging. I completed all missions and collected all treasures. Other examples that come to my mind now are Lost In Shadow (Wii), Goonies II (NES) and a few sports games.
That´s why I never give too much attention to critics. I prefer to check some gameplay videos before getting any game