For me, it's the lack of imagination.
With residency at 5 years, I've no particular problem with picking someone who's available, though I do find it distasteful to be chasing them this far out from qualification.
The bigger problem though, is the determination that a 12 should be a crash ball player, and only a crash ball player.
Far better that the coaches look at the options available to them, and try to build the best team that they can from that.
Currently, they're trying to fit a square peg into a round hole with Lawrence, who has neither the handling nor the power to pull off the role (but would be a genuine contender at 13) whilst ignoring the 12s we DO have, like SAtkinson, Ojomoh, Hartley etc. Any decent coach would be licking their lips at the prospect of moulding England's young talent. They're not the finished article, but they are available.
We don't have anyone in their prime available, because any promising 12s developing in the last decade have seen their path blocked there, and moved to a different shirt (Slade to 13, Shillcock to 15, Redpath to Scotland, Williams & Tomkins to Wales...)