It maybe more that immunity within the body fades quickly, not so much that the virus mutates to the point it's different enough. The linked article talks about

It maybe more that immunity within the body fades quickly, not so much that the virus mutates to the point it's different enough. The linked article talks about a sharp drop-off of coronavirus antibodies over time, which doesn't happen with antibodies for some other viruses (e.g. varicella antibodies are lifelong). And it doesn't sound like we really know why this might be.