The simple answer I would give is that functional programmers are harder to find, so fewer big companies want to take on the risk of FP on a large project.

The technical answer I would give is that you wouldn't want to write _all_ of an application using just FP. The higher-level code will usually always be in a traditional imperative style, since it's counterproductive to avoid statefulness at the highest levels. Almost all large projects will use FP techniques in a lot of lower-level areas (except areas with performance sensitivity.) So now the question becomes whether code that is 90% FP but is "poisoned" by imperative code at the highest levels is still FP.