The external consultant developers are incentivized to keep essential knowledge to themselves, in order to get new contracts later. The supplier's IT consultants (Bekk) don't share essential knowledge with the customer's developers (Vy).
We're incentivized to work this way because that makes the customer feel safe. Which is how we win contracts.
In fact, I think what you suggest would be an extremely short-sighted "strategy" for a consultancy. Sure, you might get the next contract working this way. But once the customer could get rid of you they would, and they would never hire you again.
At Vy, Elm is used for frontend, Java and Kotlin for backend.
Kotlin in the backend isn't a mainstream technology choice either.
So far we are not dissapointed, and in my opinion Elm is on its way to become mainstream because of these things.