If you look at the Myers-Briggs personality types listed at TypeLogic, you'll see for the INTJ type (most often associated with engineering types) as being said to often "Work at" a relationship. This is, I know, a weak argument at best, but I would toss out as something to ponder that perhaps there's a higher incidence of analytic types to dissect and improve their relationship. ...or simply inclined to remain with…
Honestly, with money being one of the leading causes of divorce, I would say a reasonable assumption is just that engineers make more of it and are more likely to spend what they do have wisely.