My guess is that it doesn't mutate as quickly as a corona virus - so our body can build a defense against it rather easily without high chances of re-infection,

My guess is that it doesn't mutate as quickly as a corona virus - so our body can build a defense against it rather easily without high chances of re-infection, just like most other poxes. It basically is quite slow moving, and will burn through its fuel in the major population centers quickly if it does peak - unlike corona which can just keep peaking after it mutates using the same people.