Because they own, build and maintain the place, sim-pel.

Also we select admin/mods by the person not the clan, that they are in those clans is just co-incidence, by time, by joining, merging, death of the clan etc.. Remember a lot of people joined Clans/merged with dm in the past. DM is still the remain of clan Merc, DL, Elite of Elite and some DW.

last fact.
When this started, moderators/admins came from EM, DL, [GpW], DW, JL, AS and much more :P