Why don't all N64 games use the Expansion Pak?
I mean, sure it was released in 1998, but the console was designed to accommodate it!
So from the start, Nintendo knew very well that they would add memory!!
So probably they didn't know how much it would be (2MB, 4MB or 6MB).
But still it was possible to make Mario 64 or Mario Kart more fluid with an addition of 2MB.