Why hasn't functional programming taken over yet?

Answer 1

Two things:

1. It just takes time no matter how good a technology is. The ideas behind FP is about 70 years old. But its mainstream usage in Software Engineering (in the trenches, in industry) is probably less than 10 years. Asking developers to adopt racially new mindsets is possible but just takes time (many, many years). For example, OOP really got mainstream usage in the early 1980's. However, it did not gain dorminance until the late 1990's.
   
2. You need people to be forced to face a technology's strength before it hits it big. Currently, people are using tools that does not not make use of parallelism and things works okay. When apps that do not use parallelism become unbearably slow; then many people will be forced to use parallelism-tools and FP may shot up in popularity. This may also apply to FP's other strengths.