In as much as regular Warframes and Sentient, I suspect it's due to their biological aspect rather than strictly technological. Infested basis, thus out of their "ball park" in theory.
Then you've got Revenant, which from what I recall of the quest dialog, put itself in reach of some kind of tendril the Eidolon uses to construct itself which went...badly. Personally, I wonder if the way Gara killed the original Sentient the Eidolon are mere fragments of, involved a notable amount of Void energy. If so, could Eidolons in some form be a cross between a Void Echo, and actual physical materials? Alternatively, if not necessarily Void based, could it be Transference usurped; Transference being about the relaying of a mind to an available body, then the "will/spirit" of the Eidolon itself could have become too much for the system to manage, corrupting what we now know as "Revenant"?
If there is more Void at play than not, could Eidolons be similar to the Lua ghosts? From there, perhaps simply having the ready availability of Sentient remains allows the Eidolon to manifest, whilst these ghosts seem to be devoid of anything they could use in that fashion.
Certainly wondering if it's a weird play on "Ghost in the Machine", at least.
Sorry for going on, any event. Best speculation I have on matters at present.