Well, i think it's because of bad Soviet economy and change in Soviet space program policy.
After USA reached out
Moon By 1969, there is no sensible point of reaching moon
at second place. Space race is for being the very first. So, USSR's next step would have been Mars or Venus. But they decided to plant a ""habitable"" and floating space station called MIR. USSR opened a golden gate for habitation of space. You know, USA used MIR in 1990s till MIR's end too.
Going to Mars or Venus would have put hard to deal with costs on slowly weakening Soviet economy. So, MIR project seemed more feasible.
After Apollo 13 accident, Moon Mission also seem too risky to handle.
Don't underestimate Russia. They might help you against China.
