Agreeing with Whopity, I think that there are two reasons for this. One is that a microlight instructor usually also does maintenance, provides hangarage, may well be managing his airstrip- so there are more revenue streams. The other is that because you can't hourbuild on a microlight towards an ATPL, the wages are not being depreciated by lots of young instructors whose main objective is lots of hours so they can get an airline job, rather than a decent income and passion for flying.
G