The main edition of Linux Mint is available with MATE, Cinnamon, Xfce and KDE. The Linux Mint Debian Edition (LMDE) is available with MATE, Cinnamon and Xfce (and unofficially with KDE, from Schoelje's work).
There are plenty of users (here's one, hi
) that give preference to one desktop over the others. Here is a good look at which desktops are preferred by Linux Mint users: viewtopic.php?f=143&t=100787
. That should probably answer your question as to why Linux Mint offers choice