Yeah it's the priority on the run game I think as set by the guy calling the shots, having a line that can execute the run game, and the backs matter too, they have to fit what you want to do.
First two things are strongly interrelated. Teams that prioritize the run will practice it more and cuts will be made around that element, so one tends to follow the other. McVay simply doesn't prioritize the run the way some other coaches like say Shanny does, though we don't know that for sure for the record without comparing camp snaps and all that kind of craziness, so it's just an opinion I have on it. Re: McVay's disciples the best of them in the run game has been LaFleur in Green Bay but even he hasn't been a routine showing high on the RB production stats, been more hit and miss. So philosophically they seem to follow his lead which is not a surprise, since both he and they have been successful prioritizing the pass.
Re: the late round picks what is going to be interesting is you have some excellent backs taken this year. Vaughn went before the Evans pick, to Dallas, and isn't a bellcow type but he would have been a really nice fit I think for McVay's offense. And then McBride, who was flat-out one of the best backs in the country, went to the Vikes some picks after Evans. Tucker will probably be a really nice pickup for the Bucs as a UDFA.
In the end though the problem the Rams have had for years now is poor evaluation of the RB position. It goes back to the beginning with McVay's regime. He maximized Gurley who was a back of great rarity. But finding consistency in a guy who can be counted on has been an adventure for him. Strangely enough though he won a Super Bowl and damn near won another one. But I think in spite of all that he probably has two Super Bowl wins if he had simply prioritized CJ in carries in the Super Bowl instead of Gurley, who was at that point simply not the same guy. That loss I think pivoted on two key things, those being the loss of Kupp to injury as well as CJ falling out somehow with the Rams staff and/or not being utilized the way he should have been.
So for me like I've been saying for years now they need to reevaluate how they choose their RBs. There is certainly room for vindication if they hit big on Evans, but even if they do hit on him how long will that be true. Point is I think they need to find their way in the identification of "their" back by tweaking that model a bit so they can do like Shanahan and find these guys cheaply every couple drafts as needed. That should be a goal for them. Not the biggest goal, since running the ball is not their priority, but one of those goals they're working on improving through.