Probably due to kernel rebuilds to deal with installation bugs/errors where you have to attempt to install more than once the same OS. Obviously Linux systems when up can be more stable, unless you install crap you don't need then it starts getting cluttered and you open a number of potential exploit routes. The NT File system gave the ability to have multiple user levels in the Windows OS, obviously it was nothing new to Linux, but then again it was nothing new to Unix either. However XP also allowed users to install to FAT32, this might have just been backwards compatibility for older drives which couldn't handle NTFS without giving out error's but also if a user wanted to flatten their user levels to a psuedo-single user. (I say Pseudo-single user, because technically there is an Administrator account that only gets used during Safemode and why people sometimes got the error message saying they didn't have "Administration privileges" while they had the Main "Administrator" named account) So in a nut shell XP was a Cross between both branches, the original Windows line and the NT branch.