[int_19h]

A big problem is that in Crimea, divisions go across ethnic lines. Crimean Tatars are a minority there, and so they get outvoted in any referendum, but for them the difference between Crimea being in Russia vs Ukraine is the difference between their autonomy and culture being respected or suppressed. One of the first things that Russia did after annexing Crimea was going after most Crimean Tatar activists and organizations under the guise of fighting extremism and separatism. So the choice is between majoritarianism and human rights, and there's no way to reconcile them.

The other aspect of it is that separatism is illegal under Russian law, period. So, ironically, while it was legal - from the Russian perspective - for Crimea to join, it is not legal for it to leave, or to contemplate leaving. Even vaguely suggesting that there should be a referendum on independence is deemed "extremism", and there are laws making that illegal (and people have been prosecuted under those laws, specifically wrt Crimea).