Because they UV light isnt strong enough to actually kill any parasite, what it does kill is algae spores, bacteria (good and bad). There are many better ways to keep algae from being an issue, and IMO the most successful reefs are the ones with plenty of good bacteria, so you dont want to go off killing your bacteria indiscriminately.
Also people tend to use it as a crutch when they should be taking better care of their fish in the first place.