It's not just because of the small cardpool, btw. They tend to make the core set eternally legal, so that it's always a good way for a new player to start. I think that's pretty clever, and it allows them to create all those basic followers and items once, and then not have to keep designing more cheap +1/+1s (or whatever). Just create all the Ashigaru Spearmen in core amd never worry about them again.
They also don't have to keep reprinting them, either, so they really free up space for later cards.