There were many obstacles on the way to SEI.
The first was NASA comitments.
The shuttle
It was ten years old only, too young to be scrapped.
But the shuttle can't go to the Moon.
And NASA budget couldn't support another manned spaceship.
Space station Freedom.
It did not even existed, but it couldn't be cancelled either, because of Europe, Canada and Japan comitments into the projects.
Next obstacle: NASA planning and architectures.
The Johnson Space Flight Center
The 90 days study was designed at Johnson. Johnson SFC boss Aaron Cohen was a good man, but, being an Apollo veteran, he thought like Apollo > unlimited budgets, unlimited goals.
Next obstacle: as Mark Whittington noted, clinton comes in 1993. Frankly, he slahed NASA budget. Bush had raised it to near 1% of the Federal expense; Clinton cut it to 0.6 %. Yes, that's a big difference. It represents some billion dollar less.
I think that, to make the SEI works, you really need to have a POD much earlier, around 1972; a different post-Apollo program.
Once NASA committed to the shuttle in 1972, no one can't stop it. Not even Walter Mondale as vice-president !
Once the shuttle fly in 1981, you can't really scrap it. It was too big an investment to stop it after Challenger.
Once the shuttle returned to flight in 1988, you have to wait the late 90's to talk about a replacement - when the shuttle is twenty years old, Ie after 2001.