If your reached the level cap in Fallout3 halfway through the game then you were obviously playing with the difficulty set too low for your gaming abilities. Be real men, Play your games on the hardest setting.
Oh and this is a fact huh?
I never played on Normal or Hard, always on Very Hard, (except for the first 4 hours when I didn't know how to change the difficulty setting).
It's not about reaching the lvl cap by "just doing the main quest" it's about playing the game the way it's meant to be played (exploring and sidequests).
So if you play the game and do some heavy exploring (a major aspect of the game) and do sidequests as well as the main quest you reach the lvl cap easily.
But no, if you only play the main quest without exploring or doing sidequest or doing the DLC's (except for BS) then you won't reach the cap.