I still do not know why they ended the shuttle program . It was headed in the right direction. But that IS politics for you.
I’d highly recommend reading the Augustine Commission report.
Old technology, was costing more each year to keep the aging airframes in service, and it cost more to refurbish the engines then to build them from scratch. It never met it’s own design or cost goals.
On every Shuttle mission you’d spend $22,000 per lb of payload. SpaceX’s Falcon series is below $2,000 by comparison.
But even before SpaceX, you could buy several Delta IVs or Atlas V launches with the cost of 1 shuttle launch. The Shuttle is to launch capability what the Concorde was to air service. A tax-subsidized extravagance.
Which is a problem if you want to do anything big again, like, say, go to the moon. Or to Mars.
The cost of the shuttle was an obstacle to those aims. Same to how it monopolized manpower, talent, and space.
Aging also made operating them inherently more dangerous. And the Shuttle is already the most dangerous space vehicle ever flown by a human.
TBH, not a fan. Given its cost, rate of failure, and the compromises made in its formation, it represents what is truly awful about the political process steering spacecraft design and priorities.
The SLS (the new NASA rocket) is shaping up to be almost as bad, partly because it’s being built from old Shuttle parts.