I think a major reason Scala flourished was a big reason it has struggled as well: it opened the doors for many people to new and fundamentally different programming paradigms such as actor systems and functional programming. Many people got excited and implemented these new paradigms in production systems without regard to the immaturity of their own grasp of the concepts (think of how many years of imperative/OO programming you had under your belt before feeling confident of setting the style for a project), the learning curve they were non-consensually committing their coworkers to, and the hiring, productivity, and morale burden they were saddling their employers with. From the management point of view, allowing Scala in the door meant running the risk of waking up one day with a bunch of Scalaz in the codebase, one grinning functional programmer happy as a pig in shit, and the rest of the team shouting and cursing.

Likewise, on top of the real problems created by ill-conceived adoption, the irrational backlash against new paradigms is real as well. I'm old enough to remember the fuss over OO programming. Many experienced, successful coders were scared that they were about to become personally obsolete. Some OO proponents insinuated — and panicked veteran coders believed — that a large number of procedural programmers would have to retire or switch careers because they were too old or not intellectually nimble enough to adapt to object-oriented programming. Major companies such as Sun responded to these fears with a marketing blitz to prove their their OO technology was simple and easy to learn and would herald a new age of programmer productivity. Programmer culture was more driven by magazines and books in those days and therefore much more easily influenced by industry marketing. The "simplicity" of Java was an invention of Sun marketing, a successful self-fulfilling prophecy as Java became so familiar that similarity to it became millions of programmers' benchmark of simplicity.

There has been no total war marketing blitz for functional or actor-based programming from a company with the same stature that Sun had in the 1990s. The emotional reaction, the resentment at feeling a step behind the future, the anxiety about whether you will ever be as comfortable and confident in the new paradigm, operate virtually in a vacuum, or worse, in the echo chamber of the internet. Whatever latent fear of functional programming a person feels, the prospect of using Scala will bring it to the fore. The only countervailing force is the (usually lone) individual pushing adoption, and they are unlikely to be up to the challenge. There is a tension between programmers who are more oriented towards business problems, or who just want to get their job done and go home, and the programmers who get excited about different and potentially better ways to express programs. They both think of themselves as humble and results-oriented and have a hard time accepting each other as being equally humble and results-oriented in their own way. They both experience the other as snobbish, condescending, and threatening to their self-image as good engineers.

Sadly, the root of the problem is that Scala is expressive and powerful enough to be a natural vehicle for these new paradigms. An engineering organization should not fear expressive power, because they should have a culture that is incompatible with a single engineer (or a small cabal) committing their peers to a learning curve without consultation, debate over the cost and payoff, and buy-in from the team. Unfortunately, there seems to be an assumption that beyond choosing a language, you have no ability to consciously choose a style. I.e., Scala-as-a-better-Java (the least imaginative approach, but not a bad one) is an impossibility. This is grounded in the assumption that any aspect of a language that can be abused, will be abused, and that engineers cannot make collective decisions about how they will work together, which are odd assumptions to make about the behavior of highly paid professionals. As someone who wrote C++ professionally for many years, I can testify that it is possible for engineers to conform to coding standards that restrict them to a strict subset of the expressive power of a language. However, it sometimes requires management authority to bring people in line -- and perhaps this is a deal-breaker in an industry where teams of engineers are supposed to be entirely self-managing.