There are many reasons, of course: incompetence of the opposition overall, lack of an appealing platform or practical solutions, limited access to media or the "administrative resource," and opposition leaders who refuse to cooperate with each other. But one very important reason is the potency of government propaganda, which portrays any non-loyal opposition as externally funded and dedicated to the collapse of the state, and the susceptibility of certain segments of the population to that propaganda.
This comes via drugoi - where commenters pointed out that this clip did not actually appear on Belarusian TV and wondered whether the person shown might be mentally ill (watch the clip and you'll see that's not an outlandish question).
*With apologies to Vysotsky.