It's just all about contracts. Cirroc Lofton was signed to a six-year, regular cast contract when the show started, so he gets his name in the opening credits every week, regardless of how much screen time he gets. Aron and Max did not have that contract - they were only on recurring contracts, not regular. Simple as that.
Contracts like that are signed long before the show ever airs, and are based on what the show creators assume they're going to be able to do with the characters. They assumed - probably not unreasonably - that as the son of the lead character and station commander, Jake would be featured quite heavily, enough to justify the full-time contract. That it turned out not to be the case doesn't change the contract that was signed. The only way Cirroc would have left the main credits is if he officially resigned his full-time contract, which obviously he didn't do. Presumably he's not one of those actors who think they have to be the focus of attention at all times.
Conversely, being kept on a recurring contract - like the actors for Nog, Rom, Dukat, Weyoun, and endless others were - means that the actors are free to come and go as they please. They don't know when the writers are going to call on them for a script - because the writers themselves probably don't know - so they keep their options open by only signing on for one episode at a time, so that they can take other jobs if they feel the need. That means that sometimes the actor isn't available when the producers want him - Duncan Regehr's absence from "Rapture" for example - but on a quality show like DS9 I imagine most people, when they got the call, said "yes."
It's not just a matter of popping in and out of the credits based on the number of episodes you appear in - there's a whole big legal matter of contracts and agents and all sorts to take care of.
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