On average two people have died at every convention I've attended (attendance in the tens of thousands). Lots of people there are old and have been holding out to see their brothers one last time, and the excitement gets too much for them. Part of the grieving process is to be happy that they died among the friends. *yawn* In the bigger ones they usually have the ambos on alert full-time for the weekend anyway...
I have family in the first aid unit at these gigs (umm, cleaning supplies and bricklayers in real life), and I'm not surprised that they don't call an ambo at first. Neither would anybody around them as they're all accustomed to looking for direction. Anybody with a clue would be pushed aside by somebody with an attendance badge.
Still it's not all bad news; something to be aware of is that this is policy at most stadiums - staff are usually trained to tell the first aid staff first, who will then deal with the situation (usually involving calling the ambos), but they know how to get the ambo up to them the fastest so they're better off doing it. Not necessarily true of Mr Bad Suit at a DC, but you get the picture.