I have no idea why they didn't have IAD or BWI as an alternate as I wasn't the person who drew up the flight plan. To be honest, there are a lot of different possibilities including but not limited to IAD and BWI had similar fog issues (not closed, but using flow control), those airports weren't equipped to handle an AC E75 at that time of night, etc. Besides, I don't think there are AC staff at either IAD or BWI (I would assume UA handles us) so that would put a great burden on them to organize buses/taxies when they are already dealing with their own operation. There are many more options out of YYZ tomorrow morning.