.NET Framework was built to support and did allow systems to have multiple versions installed side-by-side and apps to call specific Framework versions and was built to support many versions side-by-side. But that turned out to be more difficult in practice than in theory. Multiple side-by-side Frameworks turned out to be more hassle than useful, and even with just 3 versions of the Framework that were built to live side-by-side (1.x, 2.x, sort of 4.x, long story) that was often pointed to as a source of bloat in Windows.

The mistake to include it in Windows was partly why more versions weren't made (because then they'd have to be serviced for the length of a Windows version's service lifetime; because then they'd contribute way more to Windows bloat).