Big Sur

Why doesn't Big Sur download for Mac mini (Mid 2011) if the drive space is available? Is 10.13.6 (17G14042) the latest version that will ever work on this machine?

I know the machine is old but it has continued to work very well. I had an issue back when it was out of the box in 2011 and the apple store fixed it; ever since then it has worked very well. A couple years ago I put a new ssd and memory in it and it was like giving it new life. Is it finally time to part ways with this reliable old machine?