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.
I think Windows 2000 didn't have nearly as much compatibility work as XP.
I guess Windows 95 came out because NT was a lot for a 386 or 486? And also different priorities as far as compatibility.