Apologies if this isnt the right sub but sidebar indicates that events older than 20 years can be asked.
I know the collapsing of U.S.S.R is a powerful factor but, even then there was a 5 year gap between the only Buran launch and the collapsing of U.S.S.R.
Also can't modern Russia utilize them now with being a major spacefaring nation?
The shuttle was a total failure in everything except PR. The original idea was that a reusable spacecraft could be much cheaper to operate, it actually costed like 10 times more per launch. It had the unique capability to bring back large objects from space, but that's totally useless, you send the data back and let it burn up, that way is much cheaper.
The soviet economy wasn't in a great shape and they realized these problems, so they didn't use Buran.