GoT only ended 2 years ago and unlike Momoa she didn't die in the first season, so she wasn't as free to pursue Hollywood roles as he was. Most of Momoa's projects I either don't recognize or were utter dogshit (Conan remake, Justice League). Aquaman enjoys the dubious honor of being the best of a bad bunch, but given how high profile it is maybe he can be called an A-lister now. The Aquaman role will probably protect him from the embarrassment of Dune bombing.
I'm pretty sure the only Hollywood project Clarke was in that actually bombed was Solo, which she had a pretty small role in. She's coming to the MCU next year so it's not like she isn't still being offered high profile roles.