Authorities did not allow him... There's no democracy with Putin, just dictatorship. But (1) I don't live in Russia so I don't know enough facts to say I'm sure and (2) I think one of the forum rules was not to discuss politics here 
I don't think Kasparov is waiting for Karpov's visit though. Kasparov quit chess and is barely interested in chess anymore (other than making money from chess now and then). I think he would rather be visited by the opposition leader or someone else supporting him with his politics.
I read in Russian site that Karpov (WC #12) wanted to visit Kasparov (WC #13)
in the jail, but authorities did not allow him.