The big difference between Belarus' and Crimea & East Ukraine is that there were very strong pro-Russian movements in those regions. Ukraine was always very much divided between Western and Eastern parts with pro-European and pro-Russian views being dominant in those regions respectively. Crimea always had this nostalgic image of being a part of Russia & the crisis of the 90's (extreme poverty, crime, hunger etc.) became closely associated with Ukraine. Especially since in Sevastopol Russia had their harbour to station the Black Sea fleet and the whole city profited from it (Russian government sent a lot of money to sustain the fleet). So the tension was always there and Putin capitalised on that.
Belarus' is a very different country though. While they are very close to Russia ethnically and culturally, they are still a separate nation. They have their own identity and there's no real divide, linguistic or ethnical, that can be exploited here. Putin likes to support proxy forces that already exist in the country with men and weapons to make it look like it's an internal conflict – but in Belarus' there are no such groups, if you don't count the police. Groups that are created by ethnical, religious and similar reasons can be used as the basis for such intervention, but I doubt that a group is based on the fact that those people simply work together can be used as a foundation for something similar.
At least I hope so