I think the module provided a ton of reasons for why you're there, but may perhaps need the DM to close the last 5% of the motivation.
When an unnamed DM ran this module for us *coughWizardrucough* there was a very simple and effective motivation.
We had been retained by a rich noble who needed the fruit to heal his sick daughter. Our job - go buy the fruit. Ah but wait, the fruit isn't being sold any more. If we were to succeed, we needed to go talk to the goblins trying to get a fruit. And since the previous group had gone to do the same thing, we could find out what happened to them also, and there was a cash motivator from the NPC in town to do exactly that.
I don't agree totally with Piratecat saying
Piratecat said:
I think it's really the player's responsibility to create a character who is fun to play and interested in adventuring.
The statement is true in that the character should have "handles" by which they can be motivated - be that money, power, helping people, etc. However it's up to the DM to leverage those handles.
Arravis - this leads me to a suggestion. Wizardru basically had the party in service to a Greyhawk noble. This made life a heck of a lot easier overall. He handed out missions for us (no dark strangers in inns) and could provide rewards as needed. For a starting DM this might help them inject plot lines more easily.
(Of course, Wizardru is an experienced DM, and he uses it to create twisting convoluted story arcs that two-and-a-half years later, we still don't know much of what's going on.)