Because it's just packed full of useful new features, that are of use to even a total beginner. I'll tell you the 3 most useful features it has.
1) The most useful thing is that you can now have subdirectories, which makes file management far more easier. Each Subdirectory uses a free directory slot, but that's it. They are incredibly useful, since they allow you to ie store all your word processing files in one directory, and all your pictures in another. Subdirectories can contain furthur subdirectories, and so on, allowinf nice tree like structures of files.
2) Another useful thing, is that it allows you to set aside areas of memory to use as a RAMDisk. This Basically acts as another (faster) disk drive, that you can carry out normal operations on. If you have a load of files to copy, copying them all to a RAMDisk, and then to the target disk, is a lot easier than 20 or 30 disk swaps. The RAMDisk is limited by the amount of free memory you have, and can be as large as a normal disk (800k), very useful if you have a 1meg expansion!
3) The 3rd "useful for anyone" feature, is that you can now reserve extra tracks on the disk for directory information. Basically this means that you can have more than 80 files on a disk, upto a maximum of 778, useful if you have a lot of small files, and subdirectories, since you could sometimes have about 1/2 your disk space free. Every additional 20 slots uses up 5k of disk space, a small price to pay.
Not at all, there are plently more features available to those who would use them. For a start, MasterDOS keeps track of the time (if you have the SamBUS with the clock in it), and allows you to date stamp files. You can have a confirm option when using wildcards (ie ERASE "*.txt"). There are some useful new Keywords, to find the current subdirectory name, to count the amount of free memory pages, to give information about the disk, and files. MasterDOS also finally has opentype files, allowing you to open a stream to a file (ie you can use PRINT, INPUT, and the like on disk files).
So as you can see, it is far better than SamDOS!