Windows 95 ran on machines with 4MB of RAM (8 recommended). NT 3.1 required 12 (16 recommended). That extra 8MB was a significant amount of money in 1995. NT 3.1 also required twice the hard drive (75MB vs 35MB). Windows 95 offered better support for direct DOS mode access, VxD, etc. Which was crucial at the time for games and multimedia applications in a manner that NT simply couldn't match.
This was true even for Microsoft.
Windows NT/2000/XP was effectively a new operating system that was able to run Windows applications.
Microsoft worked very hard to reproduce old bugs and undefined behaviors so that old binaries will continue working on the new Windows.