For whatever reason, it seems that Quins have struggled historically to either develop players in certain positions (9 and 15 being the obvious examples)
Obvious?
England's most capped fullback, close to 400 appearances at club level, top 10 for tries scored in the premiership...
As for the difference with 2012, it's pretty obvious:
Harlequins: Brown; Williams, Lowe, Turner-Hall, Monye; Evans, Care; Marler, Gray, Johnston, Kohn, Robson, Fa'asavalu, Robshaw, Easter.
Replacements: Buchanan, Lambert, Collier, Vallejos, Guest, Dickson, Clegg, Hopper.
That squad had a lot of players a mile away from international selection and a couple about to pick up a handful of caps. JTH was capped that summer then binned by England. Gray got nowhere near for years. Kohn got a comedy Wales call at the end of his career. Robson was never capped iirc, Robshaw and Marler had only just started playing for England regularly. I think only Vallejos on the bench had any caps.
Brown, Care, Easter, NEv, Monye were the big names there but there were a lot of players who wouldn't be commanding big wages or being targets for other sides. The quality in the squad now is miles ahead of 2012; we have JEv instead of Clegg, Porter instead of Dickson, about twenty backs better than Hopper, and guys like Chisholm, CCS, Lawday as upgrades over Guest. Our first two choices at loosehead are England internationals, one a Lion, and we have Welsh internationals who warm the bench or can't even make the squad
It's a completely different era. Much more quality throughout the squad and plenty of players who would command big money in places like France because of their sustained impact in big games.
When quins finally started developing England internationals, we suffered badly from the drop off when they were called up or injured. Now the squad is far less lopsided, but it does mean some very good players aren't able to hold down a starting spot or are clearly worth huge money on the open market