I know it's an overused reason, but I really think it's clear EPL teams see the EL as a burden. In fact wasn't it the Swansea manager last season who said something like he was happy his team didn't get an EL spot?
As for the CL. I don't think Italian teams fared better than English teams there in the past few years.
That being said, I do think the EPL has been average for a number of years now, I just think the Serie A has been worse.
Right now, La Liga is a level or two above those leagues.
Fvcker