I just learned one of my favorite road races over the past 6-8 years will not be run this year. The race was always well attended, was well organized, and since it was run on quiet streets required few police.
I'd be interested to hear from current/former race directors.
Inappropriate victory celebrations?
It's a lot of work to put on a race. There is usually little reward for the effort as well. People get burned out, move on. If no one else can be found willing to pick up the slack, the race dies.
There can be more specific reasons too, e.g. a road that race runs on will be under construction. This kind of stuff can usually be worked around, but it does involve more work so it all goes back to the first paragraph.
One successful race: The local police just didn't like us and would not grant permission after like 10 years, unless we jumped through incredibly difficult and unreasonable hoops.
A small but fun race: No sponsor and the race was a money loser. After 7 years, I shut it down.
It is a ton of work to be an RD. Race day is an emotional nightmare: worries about weather, runners getting badly hurt or needing medical attention, volunteers showing up (once had twp whole aid station crews in a half marathon not show, and had to recruit people from the spectators, and send them out on the course.)