To me, it's pretty simple. After a rookie year in which Gurley got the league's attention, the absolute failure to establish any kind of downfield threat defenses realized they had to account for Gurley, but need not fear anything the Rams did passing. So, they crowded the LOS. We had nothing to drive them off.

By "crowding the LOS," I don't only refer to "8 in the box." It's not exclusively about deployment. Even if, technically, a defense has some people back, they can still be cheating up and reacting to everything expecting run. Normally, when you get a defense cheating 1 way, you burn them by going the other way. We simply lacked the passing game to do this. There was no penalty for a defense selling out to swarm the run.

You cannot run in today's NFL without at least a mid-level downfield passing game. Can't be done.

Note: by "downfield" I simply mean passes attacking 10 yards or so upfield.