I can think of several reasons, most of which have already been touched upon.
1. Combat is too luck dependent. I prefer more tactical options that the slightly higher levels bring.
2. Been there, done that: I’ve played those levels more than any other. Too many campaigns die early anyway.
3. You can write a more varied background, if you are so inclined. I personally don’t care for the ‘fresh young adventurer’ paradigm. In fact, I’m not interested in a character’s early career much at all. I prefer ‘…and this is where we are now’ to ‘this is how it all started’.
I actually prefer slow advancement. I would be fine with leveling up every 20 sessions even, as long as I don’t have to deal with the monotony of those first few levels.
For the same reasons, I prefer not to run games that start at levels 1-2, either. I've found that starting at 3-4 is really my preference.