I think it's hard to wrap all those hobbies up into one "reason" for the shift. One though is that kids probably see a lot less ships (in real life and film/TV) than they did 20, let alone 50, years ago. Kids will react to what they're told are cool and young adults will continue the hobby they had as kids.

That being said, the same place I buy my monster and spaceship painting equipment from seems to still do a decent trade in benders and slips.

And size is still an issue, the smallest ship I could build is about the size of the largest "miniature" I have.