My first con, I only ate a flimsy Taco Bell taco a day and whatever I could munch on from the Dealer's room-Granted I only went two days though.
My second con I came prepared, I brought two cups of ramen and two cans of Chef Boyardee to last me for three days-In which I had to eat the Chefs cold due to lack of microwave.
My experience has been that there just isn't the time to stop and eat, and sometimes eating adequate meals in cosplay is hard work. There are the worries about getting food on your cosplay(taking off the cosplay may take more time than the cosplayer wishes to take), or messing up any makeup you may have to wear, and some go above that into the whole 'would this character eat that?'.
Some cosplayers also may not eat much in their daily lives, making not eating during a con relatively normal.
Currently working on:
Wendy Marvell-Fairy Tail
Armin Arlert-Shingeki no Kyojin/Attack on Titan