There are just too many DRGs. I mean it is one of FF's more iconic jobs. Also people probably prefer to stick to roles that they enjoy.. (melee, ranged, tank, caster etc) rather than make the switch.. and DRG has a pretty loyal fanbase (both bandwagoners and not).. Plus it is just a strong, well rounded class.
NIN is still a good class, but is in more of the supportive DPS role out of the 3x melee.
I believe it's mostly because of how much of a following DRG has, and how many are committed..
On top of that, NIN is newer than the other 2 melee.. Some people just get too comfortable with what they are used to.
Could be more to it though..
*EDIT*
Right... My signature is of my character in 2.xx as a DRG.. Hah! Just realized how this post may sound..
I actually made the switch to NIN on release of the job.. But I think my point still stands.
