It's even a bit more complex than just sourcing a particular type of food.
You also have to make sure that it all arrives at the same time as the rest of the catering, and that it is allocated to exactly the right people.
I can imagine the disappointment of those at finding out that there are no arrangements to provide Halal/Kosher food, but I think it would be somewhat worse if promises were made that for whatever reason couldn't be kept, because someone accidentally ate someone's Kosher meal, specific meals did not meet expected standards or delivery was delayed.
All entirely possible when you're catering for >1000 people.
As people have mentioned, it's contextual. So while sourcing Halal food in Islamabad and Kosher food in Tel Aviv would be easy and cheap, sourcing either in Mexico City is somewhat harder to do.
I am sure that the local bid team will do their best to provide information about where attendees can get hold of particular types of food.
Best
Stuart (User:Battleofalma)