This baffled me at first, too, but I've done some investigating about the fact that the adventure as written just looks like a bunch of rooms with stat blocks. As a previous poster mentioned, B2 was originally part of the Basic set and intended to do most, but not all, of the prep-work for a beginning DM. Thus the gutwork of drawing rooms, placing monsters, etc. is done but the creative work of coming up with motivations, reasons, etc. isn't. This was meant to help teach beginning DMs how to design dungeons, and is sort of represented by the various adventure hooks and reaction suggestions in the 5e playtest version.
The other piece of the puzzle is to remember to think about the Caves as a living, changing environment. The idea of monsters sitting in rooms waiting to be killed is just as silly as it sounds. To give a simple example, our playtest group snuck about the hobgoblin caves, going room by room until we found some prisoners. We spent the next day taking the prisoners back to town and resting up, and when we returned the entrance to the hobgoblin caves was clearly and heavily guarded. The monsters knew somebody was attacking them, and reacted accordingly. It's always good to sprinkle adventure seeds into the dungeon to give the PCs a reason to delve it other than treasure, but the plot arises naturally from what the characters do and how the monsters react, rather than being set out clearly from the get-go.
I've heard an argument that there is an implicit "plot" in B2, that the Shrine of Evil Chaos draws monsters to it through evil magic brainwaves or whatnot, which explains why you have so many creatures living in relative harmony so near to each other, why more may come to augment or replenish their numbers, etc. Nevertheless each monster or group is individually motivated, and it's an uneasy peace at best.
Lastly-- the wizard pregen can prepare Comprehend Languages if he wants.
