I know 75s need alot more TLC than A320s or 737 NGs. Maybe EI are leaving slack in the schedule to allow for line maintenance to fix the daily snags these older aircraft run into.
Making a 757 short haul route profitable is harder than the newer narrow bodies. A 757 to CDG would have expensive fares, which would only suit business people wanting to fly direct to Paris and those connecting through CDG. Many point to point would use either FR to BVA or fly through DUB.
Also, with EI now IAG bound, I don't expect they'll want to funnel pax through a rival's hub as opposed to their big 4 (DUB, BCN, LHR, MAD)