SSVP, a probe-and-drogue docking system, is the main docking system used by the Soviet/Russian space program. Ever since the first Salyut 1 mission, only three Soyuz spacecraft, Soyuz 16, Soyuz 19, and Soyuz TM-16, did not equip the SSVP docking system. Mir and the Russian segment of the ISS also make use of SSVP to attach space station modules.
Kristall and Buran notably depart from this tradition. Both were to be equipped with APAS-89 ports, an androgynous docking port modified from the earlier APAS-75 used in the Apollo-Soyuz test program. Kristall had an aft and a radial APAS-89 port. The aft port was originally intended for Buran and later used by the Space Shuttle.
What cause the Soviet to rework a docking standard they only used once instead of using the tested SSVP?