Golden Sun I feel has so much potential to become another tent-pole JRPG series under Nintendo's cap. My guess was just that the GBA's short lifespan, coupled with literally all attention from fans being towards Pokemon *raises hand* may have contributed to it not being as big as Nintendo wanted. And Dark Dawn's poor reception SO LATE into the DS's lifespan was extremely bad for it.
My ideal revival is a full-fledged remake of the two GBA games into full 3D games as they were two halves of a whole game (surely they can reuse Mario Tennis Aces' engine if need be for ease). That way they have proven games to build off of, and hopefully they won't ruin what folks liked about them (seemingly unlike AlphaDream with their M&L remakes).
My guess is that the series was screwed over the change of leadership. Iwata wasn't very keen on supporting it.
Later 90s / early 00s was a weird period for Nintendo, with Yamauchi funding a lot of projects from smaller companies. We got the likes of Custom Robot, Starfy, Tomato Adventures, Kororinpa, Magical Starsign... and Golden Sun, from Camelot, which was already trusted with Mario sports spin-offs.
Then Iwata became CEO and restructured the whole company. He wasn't really interested in funding those projects. Over the course of his tenure, the smaller companies become dedicated developers of Mario spin-offs (like Alphadream), support studios (Brownie Brown becoming 1UP) or they simply stopped being associated with Nintendo (e.g. Noise).
I think (and that's 100% a supposition on my part) Iwata wanted Camelot to focus on Mario spin-offs. I guess Camelot wasn't very happy with this and after a 7-year partnership with Nintendo,
in 2006 they announced a Golf game for Yahoo PC service. The game was never released. In 2007 they released with Capcom We Love Golf on Wii, which was very good but kinda flopped.
Then in 2009-2010, they went back to Nintendo with the New Play Control version of Mario Tennis GC and Golden Sun Dark Dawn. Despite being a port, Mario Tennis sold very well, while Golden Sun was a big flop. The series already lost its momentum and Xenoblade, released the same year, became "the" Nintendo flagship RPG franchise.
Nowadays they aren't big enough to develop a big reboot of Golden Sun, and Nintendo seems happy to use them as a Mario Tennis/Golf factory.