There is a lot of RNG involved. I think that was the Sector that took me about 6 weeks to unlock for my FC but over 3 months to unlock when I was helping a friend's FC with their workshop. I don't know if the community ever managed to determine if any of the stats have a specific influence on sector discovery beyond the minimum Surveillance needed. I've seem some suggest it's influenced by Favor just like loot double-dips and others swear it's Retrieval. Or it could be none at all.

All I can say is keep trying.

Submersibles are the new "airship" content. They should be considered as the same thing (exploration voyage content or whatever term you want to use). That's why the limit on vessels is shared between the two. Since we know there's going to be another floating island zone in Endwalker, SE might choose to move back to expanding airships instead of subs next expansion. And who knows? Maybe the last upgrade will get them to the moon as you suggest.