The primary reason I can think of is that I got the idea for a wise old turtle druid and I don't know when, if ever, I will get to play him. Maybe I saw a character from a movie who inspired me. Maybe I found a cool mini or picture online. Maybe I really like turtles and I'm bored or elves and dwarves. That's true for a lot of players who only play in one game or with one DM (and who rarely even switch worlds to give different options). If that DM is the only DM I play with and that's the only setting he ever runs, I will never get the chance to play that character.
I think my flaw on this thinking is assuming that I need to be DM shopping to play the character I want. That just because I want to try something new, I need to abandon my current group and find a new one and hope that DM is more permissive while also adjusting to a new group and DM who will have different quirks. Hell, I may need to leave a group of friends to game with strangers just to try out my idea. (Then again, if my group would rather I leave then play a Tortle, I'm pretty sure they weren't friends)