I think proper resting is only part of it, all English clubs still employ the rugby league defence and still pay little attention to the ruck, Pro 14 teams actively challenge at the ruck and we are getting left behind. Also Pro 14 teams just seem faster. Leinster are running onto the ball at speed every time with the 9 whipping the ball out of the ruck quickly, we seem to do everything at a slow, plodding pace in comparison.
I think English rugby is too stuck in the past of slow, set piece style games. Only Sarries really try to play free flowing rugby offloading the ball out of contact. The attack from the rest of the teams is quite predictable.