Emet-Selch stated in the ShB story that Lahabrea became weaker because he so often switched bodies, while Emet-Selch prefer being in one body for as long as possible, and in the same form. That might be why Zenos could overpower Elidibus so, but it doesn't really account for Zenos also having switched bodies, so I don't know. We'll have to wait and see in future story patches I guess.
Quote/banter during a MSQ:
Elidibus:
Be it for a year or a millennium, I prefer to retain the same form until my duty is done. So, after arriving here in the First, I fashioned some hapless body into the you see before you.
Though as your friend over there can attest, there are those of us who forgo such alterations.
He was ever the rash one, Lahabrea. Jumping from vessel to vessel. Never heeding the toll it took on him.