I think that is pretty normal for a good aligned party.
The rescued prisoners convinced them they had to help them go back to Cauldron and that they were not prepared to brave the dangers of the Darklands.
When they were coming back to Cauldron, the beholder appearance and later on the goblin attacks made them forgot about the missing prisoners.
But then on Chapter 4 they are set to visit the Darklands (Underdark on D&D) to rescue Zenith. Instead of making them reach the Kuo-toa lair through the open land, I decided it made much more sense to do the walk through the Darkland tunnels. And so, we reached Bhal-Hamatugn through Darklands trip from the Malachite Fortress onwards (the fact that Davked gives the mission and that there are some keys for the Darklands demon doors makes me think this was the original idea). This let me introduce a sidequest where they rescued most of the remaining prisoners from duergar slavers. The players were grateful to have a closing for this part of the story.
From there on, I had both Crazy Jared and the pit to happen in Darklands caverns. All the trip had more sense in this way upon my view and I think everybody enjoyed it. They still remember their trip through the Darklands.
Going to copy and paste this from a post of mine from ~5 years ago:
My party had to flee from Kazmojen their first encounter, as he defeated them very easily. As they were fleeing, Lord Orbius made his appearance and took Terem, but I had Kazmojen go ahead and sell the other children to Pyllrak. The party was quite distraught when they finally defeated Kazmojen and found a receipt on him indicating that "Pyllrak of the Darklands" had purchased them. They found a discarded journal in Pyllrak's room that indicated his plans for reselling the children in the Darklands, and so the second chapter of Shackled City turned into a chase through the Darklands as the party tried their best to catch up to Pyllrak. It turned into one of the best parts of the entire campaign.
I spaced out the kidnappings a little more so that it wasn't as rampant as the module made it out to be. With that buffer I made it clear from Kazmojen's records that the last sale had occurred one month prior, so the trail was long cold and the group's chances of tracking down slavers in the Beneath (homebrew Darklands) were Slim and none (and Slim had just left town). With no ranger in the group, they came to their own conclusion that it was hopeless.