The visitors enter the zoo and the game generates a list of what they want to see and my understanding is that it prioritizes the most recently added habitats (i.e. those are more popular). Click some guests at the entrance and look at both what they are interested in seeing in terms of animals, and also what they are currently headed towards.
Both of those pieces of information should give you a general idea of why they are there and should be able to help you figure out what to do to change it.
Scenery helps make things attractive to guests, and (unfortunately) shops are a big draw as well. If you want an area of your zoo that is less populated to attract more visitors you need to put down shops along the way at pretty regular intervals (every 40 to 50m works for me) and NOT use food courts. Spread the food and drink and souvenirs around and use them like bread crumbs to draw people through the zoo. It's unfortunate that the game works this way, but it does.
I had a Nyala habitat in my current zoo that was not getting any visitors (it was off the main paths a bit on a side path)..I added a drink shop right next to it and it has been a popular habitat ever since. (This was actually intentional, I didn't want it to be popular until I got 1-way glass).
I had an idea today in my zoo that I haven't tried yet. Modifying an old habitat by removing the gate and re-adding it could put it back to the top of the list in terms of 'newness". Of course I'd need to re-add it to my work zones, but it's a small price to pay if it revitalizes old areas of the zoo. I'm not sure if the age of the habitat is what makes them appealing, or if it's the newness of the animal itself...going to test to see at some point.
As far as the popularity of your bisons and pronghorns, it seems to me that any habitat with multiple types of animals is really popular...even when they are older. I use that to my advantage as well by spreading the multi-species habitats throughout the zoo and putting them at longer distances from the entrance so that it forces folks to walk through the zoo to get to them and gets activity at all the shops and habitats in between.
In my current zoo, I have 3 really popular habitats like that and each is at a different compass point. So if the entrance was "south", I have my African Safari 1 (Ostrich, Zebra, Giraffe, Sable Antelope( to the far east, my African safari 2 (Thomson's, Springbok, Buffalo and Wildebeast) and Little India (elephant, rhino and peafowl) to the far west and Planet Lemur to the far north. The whole rest of the zoo is enveloped by the radius made by drawing a circle to connect all of them.