I am not sure why that is, but I suspect it has mostly to do with the level of most user's experience with programming, the fact that functional programming can solve 99% of problems the Arduino is used for, and the "straightforwardness" of functional programming (making it easier for beginners).
Add to that the fact that one can't stray to far from the hardware when programming the Arduino and the limited resources, specially the limited RAM makes excessive use of object-oriented sometimes problematic.
Korman