Several reasons:

1) Coil is more on a level with Alexander/Omega Savage than with the NM-versions of those. So it was never big DF-content to begin with, because you just couldnt - and for most tiers probably still cant - clear it in DF.

2) It offers less rewards: Alexander is still beign farmed for Animus-Light, because specially A1S and A9S are quick and easy fights and give a lot of light. Omega drops music and gear thats still slightly relevant for jobs you want to gear at level 70 (...very, very slightly, I'll admit). There is also replica-coil-gear that can be crafte, sold on the marketboard and aquired through PotD-drops, but for both Alexander and Omega you need to do the fights if you want the gear.

3) Alexander, Omega and Eden NM are content people "actually do" - as in: its content you queue up for, content thats designed to be queued up for - a bit like my first point: The NMs are designed to be DF-content. Coil wasnt.

...I also doubt that Alexander is run frequently outside of light-farm and the same will go for Omega (if not already, then soon). Eden will be farmed for the gear for the moment, but will see less activity once the next tier is being released.