There were more longer (7-night) sailings out of Galveston a few years ago - around 2016 and 2017. I don't think they sold particularly well. Factors likely included Galveston being a new market for
DCL and people local to the area not wanting to spend the vacation time or money to take a long cruise for their first time, and Galveston not being a huge draw for experienced DCL families. (People seem to strongly prefer cruising out of Port Canaveral over anywhere else due to the proximity to WDW.)
I think that since those early years in Galveston, Disney has switched to mostly much shorter sailings, and very few if any of them go to
Castaway Cay.
We did the 7-night Bahamas itinerary from Galveston in mid-December 2017 and it was great! I'd love to see that itinerary come back, but I don't think Disney was able to price it high enough for their liking.