Hmmm.....maybe becase he's been married,and it shows respect since he's the boss of the Krusty Krab,instead of being disrespectful and calling him"Eugine!"
In one episode when he sold the Krusty rab and t was made into Krabby'O mondays,he didn't want to be called Mr.Krabs anymore,only Eugine.Its probally because he WAS boss.