I started playing this game since TS1. For me during that time (I was a teen), it was a very unique and interesting game where you can control a pixel made creature and it can even have families, die, etc. It is a different kind of genre (during that time) because I do play lots of other games as well, like Harvest Moon, Theme Hospital, but mainly PS games.
So, TS2, TS3, and now TS4. I play it out of my love for this franchise as I grew up playing the game (like how we love other games/franchise when they launched different version or upgraded version of the original game. Because I was too immersed in the game (especially TS2 at that time - I can play it for more than 24 hours per day or even before the exams), my brothers once told me that I like to play it because I want to escape the reality..but that is how most games are.. worse, they (my brothers) populated my neighbourhoods with their weird creatures :s (for TS2, we have a family shared PC) It's true sometimes, but now, as I am older, I like it because of my memories, and the creative part of it..
p/s : If you asked around, you will always find someone who played it once during their childhood..at least those that I met