Originally Posted by ESpen36

The real answers: late catering and boarding.

Thus, by the time the LSG Sky Chefs truck (or Gate Gourmet truck) pulls up to 1R, it is quite possible that the next round of pax might be boarding. The Purser or lead FA has to check to make sure that all catering elements are there. By the time everything is loaded, it is almost time to go! Furthermore, the FAs don't want to impede the boarding process (especially on narrowbody) by trying to move around in the aisles with drinks.

For all of these reasons, pre-departure drinks, especially other than OJ and water (and champagne on AFS/IFS), are becoming scarce.