I have always been under the impression that it was due to the fact that their eyes tend to protrude a little more than other dogs. I guess this means they tend to get dust and other little things in them easier. I'm not sure though. Good question.
"We should work harder at being who we should be than trying to hide who we are."---Unknown
because they are so close to the ground they are more prone to getting dust etc in their eyes which make them weep - also sometimes their tear ducts cant cope because they are so small or they may be blocked (so next time ur at the vets u may want them to check them) very cold or hot or sudden climate change seems to make them weep alot too