Just kinda thinking out loud here, Leonid, but it may be due to the fact that enemas may be the last unexplored 'taboo'. Adult film producers and directors may be a bit wary of 'going there'.
The Collector has a couple of videos, "Sabrina & Drake" and "Lea & Scott" that feature oral sex and manual stimulation during an enema. However, I can't recall ever seeing any adult films which feature penile-vaginal sex during an enema. That doesn't mean that there aren't any, just that I haven't seen them.
It'd sure be nice to see films portray enema sex in all its glory.