So why did this happen?
There are three ingedients.
Second, the Full Stack Developer has become the expected norm. People don't focus on just one area anymore except in extremely specialized circumstances. Instead, they opt to learn a bit about every part of the stack. Should something out of their depth arise, they simply look it up. When building an MVP, such an engineer is likely good enough.
And finally, productivity is highest when you don't have to context switch. You have heads down time, focusing on the task at hand, banging out the solution one line at a time. This doesn't happen immediately. It takes a few hours to get into the flow.
Are there any other programming problems looking for solutions by new languages?