Probably because there's not as strong a draw for those IP's as there is for Jurassic Park. Haven't most of the recent Land Before Time films been direct to video? They aren't going to spend the dollars needed to develop new rides unless they know they'll make their money back tenfold - Jurassic Park covers the dinosaur draw pretty well and Universal doesn't seem to be looking to cater to the younger kids as much as Disney does.
And it probably costs Universal a bit more to use IP from Amblin than some of the other stuff, they don't own Amblin they are just minority partners. So they would have to be pretty confident in the guests flocking to it for a ride to make sense to build and staff. If there weren't so many rights issues with game characters, I could see them *maybe* building a Ready Player One VR experience.