IMO, most of the difficulty comes due to developers having full-time jobs programming in languages they already know. Learning a new language takes a few weeks

IMO, most of the difficulty comes due to developers having full-time jobs programming in languages they already know. Learning a new language takes a few weeks of focused effort, then a few months of practice using the language almost exclusively to really solidify it. That's almost never possible while employed: most employers aren't paying their developers to spend weeks-months to learn a new language they aren't already using. It's only in extreme cases where a business decides to change for some reason that they'll pay for the training and require devs to take the time to learn & practice. It does happen (my employer did this, swapped the backend from Java to Go, and paid every dev, even non-backend devs, to take training) but it's rare and expensive so there needs to be a very good reason for it. Many developers (if not most, I have no real data) don't program much outside work, and even if they do they'll keep going back to work and reinforcing the existing language's style in their minds. It takes time and effort. Maybe not a whole ton of time, especially if the languages are of similar paradigm (OO, functional, procedural, etc), but that's time and effort many wouldn't want to put in on their own.