Learning and mastering a new language and applying it to an existing code base is quite a lot of work. That takes a lot of time away from other things they might be working on.
I think these are mostly (overwhelmingly?) volunteers, so you can't just assign them work. You need to convince them to want to do the work. Doing this shouldn't be dismissed as nontechnical nonsense... it's a very significant hurdle, and one that's directly on the critical path.
BTW, naming and shaming and internet pitchforks are guaranteed to make people not want to do the work, so unless you actually want to kill rust-in-linux, you might want to shut that down.
(Also, I wonder if there isn't a better way to bring memory-safety to the linux kernel. One that is just as effective as rust but less disruptive. The hostile us-vs-them dynamic building up around RiL might torpedo the whole thing, so it might be worth looking for effective alternatives.)