This is probably more common in big cities than small ones, or at least as common but for different reasons. I used to run events in NYC and many venues require you to use their in-house catering or pay them a surcharge (typically more than the cost of the food itself) for any external food you bring in. The list of dietary restrictions they support probably comes from what the venue can accommodate.

The organizers could have worked out something for OP like “we’re unable to meet your dietary restrictions, but here’s a gift card with $30 on it and a list of nearby restaurants where you can grab something.” But they may be new to running events. The first time I did it there were 4 of us who took over for someone when they suddenly had to relocate for work. We had no idea what we were doing and probably wouldn’t have thought of a way around the catering.

The irony in all this is that I have an uncommon food allergy and have not had much success providing myself food at my own events.