No, sidequest/mainquest is the whole issue.
The low level game has lots of sidequests, and can therefore be a sandbox with lots of player freedom. The high level game is pretty much completely focused on mainquest (plus petty management issues) and is thefore much more of a railroad.
Basically, two completely different styles of play.
This isn't true at all. I don't understand why you are asserting an objective truth here when there are lots of people telling you their experiences are different.
First of all, it isn't a video game. "Main quest vs side quest" is a meaningless distinction in an RPG where the GM controls the pacing and distribution of quests. Unless your game involves NPCs hanging around town with exclamation points over their heads, asking the PCs to bring them 8 apples, it shouldn't be an issue at all. Some games have a more focused structure and others are more broad.
Second, even if you structured your game that way, level has nothing to do with it. Low level characters can focus their energies on beating the BBEG just as easily as high level characters can trot off in random directions looking for interesting points on the map. Nothing about high or low level demands a certain style of play.