Money and lack of a mission.

After the collapse of the USSR the Russian space program was facing severe budget woes. Walt Cunningham's book The All-American Boys goes into some detail about this (although fair warning, the bits about the Russians are a bit ranty and have an obvious bias.) Roscosmos's budget was a small fraction of NASA's and they could barely afford to keep the Mir space station operational with relatively cheap Soyuz and Progress vehicles. An additional complex and expensive system like Buran was totally unsustainable for them.

Re: lack of a mission, what exactly would Buran have accomplished? With the benefit of 20/20 hindsight we know that the space shuttle was a terrible launch platform for satellites, and that unmanned rockets were far more efficient for that job. A lot of the science missions that the shuttle did were of dubious value, and the Russians already had Mir as a research platform. The only really unique and valuable thing the shuttle did before ISS assembly started were the Hubble Space Telescope repair missions.