IMHO, as terrain is out of our jurisdiction to fix and YES it was the culprit for almost no traction near the top edge, tires are to be blamed MORE than any other factor... mere 5mm wide & deep grooves, lots of slick surfaces and tarmac friendly tire pressure was apparently the cause of little traction. Enough torque & power were available all the time.
Though weight has increased considerably, it sometimes helps to gain traction as well in slow maneuvers. But mostly, its always good to have a lighter vehicle to have a quick momentum & less weight to drag on uphill climbs.