I volunteer with my local humane society who does take in rabbits and other small animals.
I can guess the reason why they dont accept rabbits-they dont know anything about them and they dont have the staff to care for them.
Be prepared that if you do bring up them accepting buns they will most likely ask that you help with the care of them. If they dont have the staff you will end up being the caretaker of the buns. One other local HS around here accepts rabbits. Both of our HS are fairly large and the reason they accept buns is because the rabbit volunteer group is large enough to care for them.
If our shelter didnt have the rabbit volunteers that is has they would rarely take in a bun. It is just easier to say they dont accept them than to bother with an animal that their vets dont know how to care for and they dont know how to market to get it adopted. All cages must be cleaned every morning and they rarely have the manpower to do this for all cats and dogs, let alone buns.
We have had bunny illnesses get missed because our vets dont routinely care for buns. The area vets are crazy expensive and even with a discount its still a horrible cost to the shelter.
I dont want to discourage you. It would be great if you could get the shelter to accept buns. Because I volunteer at my shelter they expect me to be there three times a week to clean cages and socialize buns-I am even on their schedule which isnt always a good thing.
I recently got a full time job and they arent too happy with me right now.
The other thing is there are a lot of paid staff as well as volunteers so sometimes one group doesnt know what the other group is doing and it can be really chaotic at times.
A bunny group would be great. We have one here and we coordinate the bunny care at the shelters making sure they have hay, pellets and veggies. If there was a group of you the shelter would be much easier to approach about rabbits.