It is because if makes transfer of a country from Russian sphere of influence to the Western one less lucrative.
West gets very good deal with Ukraine since Russia is stuck with the obligations to finance pro-Western anti-Russian regime to the tune of BILLIONS of dollars. So West gains and Russia pays.
But the victory in Belarus will mean huge expense and no gains for the West while saving expense (plus providing leverage in form of debt) for Russia. That way the color coded revolution might lose its steam.