Soviet technological development was a very weird universe. They had a lot of very talented scientists and engineers and were certainly capable of coming up with many novel innovations on their own. However, within the communist soviet system politics ruled everything. Whether or not a project would receive support was utterly dependent on political factors, which could be affected by prior success of course, but not always. Soviet leadership put a heavy emphasis on espionage in guiding technological development, sometimes without much critical thinking. This goes back to the days of WWII spying on the Manhattan project and copying of captured B-29's into the nearly identical Tu-4. But you see many other projects in the same vein, such as the Tu-144 (Concordski) and of course Buran.

The fascinating thing about Buran is that the Shuttle was built with a really very bizarre set of functional requirements that imposed steep demands on the design of the vehicle but which turned out to be mostly useless in a practical sense and were not actually meaningfully used during the lifetime of the vehicle. For example, the large wings on the Shuttle give it a very high cross-range flight capability during re-entry, but that was only a requirement because it was thought that the Shuttle would perform a particular kind of military mission. A polar launch which would deploy a satellite, or perform other activities, in one orbit and return to the launch site. Such missions never materialized, and such missions were unlikely to be valuable to the Soviets at all. And yet that requirement dictated the size of the wings which forced the frame to be larger and heavier, which reduced the payload, and which created a larger area that needed thermal protection (an issue which contributed to the loss of Columbia, for example), and so forth. A clean sheet design without such a requirement should have had much smaller wings and thus be a more capable and more reliable design, but Buran copied the Shuttle too closely and ended up with all of the faults of the Shuttle design without a good reason for them.