what to explain here? Everything is well known. Chechnya wanted independence and it was bloodily squashed. Pretty typical story. Russia isn't a modern Britain where you can have an independence referendum. Russia is a very autocratic imperially ambitious state. Just publicly talking about separation (check the recent set of laws incl. the "public mass media" law) will most probably lead you to jail.

Of course, like pretty much any other power, the Russia uses double standards - one internally and another externally. So right of self-determination of Crimea has existed only until it actually joined Russia :)