I’m not so sure the Buran Shuttle was so much unsuccessful as it was too expensive for them.
For that matter I’m not sure ours were worth the cost considering the fact that we’re going back to the simple heavy lift rocket style.
What exactly wrong was with their space shuttle? AFAIK it was a first ever successful robotic aircraft. No crew, no remote control, it was building it’s own flight plan and all with about 128 bit of computing power.
The problem is shuttle is not a good way to travel in space. Vehicle itself is costly and a lot of payload. Conventional capsules aren’t that sexy but ten times more efficient.