It seems the Luna expedition was on a downward spiral. Everybody was tired and hungry; the necessary supplies were not inward bound as needed. When Major Suaz arrived in Mobile Bay, the group was all together once again. Settling the Gulf Coast was secondary, the king’s top priority was now colonizing Santa Elena, so heading home was a good idea, start anew I say.
Here is an article from Archaeology Archive describing the remains discovered of one Luna’s ships.
Diving into History: Tristan de Luna’s Ships