From a hardware level, note that the Lisa has an MMU and the Mac does not.
More generally, this was because the original Mac was designed to be an appliance computer, more like the original iPad (which, you will
also note, did not have user-facing multitasking). The computer would become, totally, what you wanted it to be in that moment, and in the next moment it would be something else.
IBM and Microsoft was able to do it while maintaining compatibility with OS/2 and Windows NT. Why not the classic Mac OS?
Microsoft got a leg up with NT from DEC IP that they perhaps strictly speaking should not have had. OS/2 is an IBM invention. Copland was going to be Apple's equivalent, but it fell on its backside mostly because at that time Apple were basically incapable of releasing anything in a finished state, or indeed finishing anything. The difficulty was largely organisational, rather than technical.