The reason why Disney made movies based on lesser known titles such as Guardians of the Galaxy was because other studios had the rights to the more popular and familiar characters and groups. Universal had the rights for Namor until relative recently, and maybe still has, since Kevin Feige said the whole thing was "complicated". If Namor had belonged to Marvel/Disney in the first place, perhaps they would already have made a Namor Movie. In the meantime, while Disney is working out the rights for the character, Warner was free to make Aquaman.
Both Namor and Aquaman have a lot in common; they are the rulers of Atlantis. They have super-strength, they can live both on land and in water. Their are hybrids; their father is a normal human, their mother from beneath the Ocean. They control giant beasts.
If Disney had made Namor first, it would have made Aquman look like a copy. Just like The Shadow was considered a Batman rip-off by some, or that John Carter was inspired by Avatar. Sadly a lot of today's audience is not familiar with the background stories when based on comics or books. If Disney had made a Namor movie today, the audience would think "didn't we already see something like this in Aquaman?".
So, since Warner was first, they had the sandbox all by themselves. How Disney is gonna solve the Namor problem without making him look like an Aquaman copy (yes, I know he is older than the latter in the comics), remains to be seen.