I don't see the link. For starters, different pantheons. Also, I'm not convinced the Mothers are utterly incapable of visiting Earth. They certainly don't do it regularly, but that could be chalked up to them not wanting to warp reality, or Mother Winter not wanting to travel without her "walking stick" and Mother Summer refraining from doing so for the sake of balance.
*snickers to self* different pantheons? My dear Sir we have in the Fae make up no less than 3 distinct pantheon's all colliding into one, lets not forget Hecate is Greek, and the two have a direct connection to the Queens based upon Hades own defense systems. Odin is Norse but is mysteriously subplanted into a largely irish mythology which Bob's fairy wars directly links to the Vanir and aesir. Separate Pantheons indeed... and besides the noticeable lack of the main greek/roman deities who later were reformed(a reforming of identity would certainly be a Godly death/twilight btw) We have Hades talk with Harry to point to a little ahh Nemfection apparent in their continued actions and we Have Thomas's soulgaze(to which I've already well connected the heirs of the sons of Ares and Aphrodite) to show the Greek pantheon is perhaps in absentee now. It suffered the same Decay as showcased in Denton's actual mind. But Thomas's soulgaze was not purely a representation of his own mind, but of his identity as a whole.
*why do they actually warp reality though? what about them disagree's with it? Look at another example, Shagnasty, whom is an angelic ghost who violates said rules by staying on earth, he warps reality only slightly because he's not quite strong enough for more, but his fundamental existence here is described through DR as wrong in the extreme, and that's before we find out we keep such creatures amongst the rejects of reality. Shagnasty is directly described as a ghost in order for the dine, an excorsim to effect him, he's also, by the very story of the skinwalkers, is describes as from a different age where he was supposed to have 'moved on' and refused. Bringing it back to the spirits who effect reality and fate whom are not supposed to. MW has no Rent, no GreenCard and has been placed on the blacklist of do not travel, for a very good reason. A cosmic level reason.
*which is NOT to say she won't one day
walk the land, and for good reason. An oh my stars and garters, the day she stands up unaided by stick or escort to walk the land is a day I actually look forward to seeing in all it's terrible glory... and sadness as we watch the one who follows. ;-(