Get a decent static analysis tool for their IDE, that points out places where they can replace a gommy, ugly loop with a higher-order function.
In my opinion and experience, Resharper is the easiest way to learn LINQ, before you even start looking at all the other dumb, inconsistent and error-prone stuff that people will do because it compiles, blissfully unaware of all the foot-guns it contains.
Sure, they could be taught. But they're busy being taught other things - Android, say, or the latest JS framework. So there's an opportunity cost to teaching them - it takes away time and attention from teaching them other things, things that may move us closer to our business goals. So in practice, yes we can teach them, and no we won't.
And now that one line of code can decrease clarity for the team.