Interesting question... My tastes haven't changed much, but my approach has.
The main reason I started chatting is to find models who will do shows for a fetish I have and especially in the beginning all the shows I did were fetish shows. I chat with girls in order to have them fulfill my fantasies (and to get to know them over time, if we hit it off) and not to socialize in a group, so I've always been a private show type of guy and that hasn't changed since I started chatting around 2006.
I find that public chats don't really give customers the freedom to direct the action unless the rest of the room is in agreement. They are more of a fan club / hang out environment, where you just go with the flow and the general goal of everyone in the room, which doesn't really work for me since I want something very specific and different from what other guys are after. In other words, I don't want to pay a girl and then have to contend with other guys who want to see her doing something different.
My fetish chats are long and therefore expensive. They are also difficult to do compared to regular sex shows, so sometimes even if a girl tries her best it doesn't work out for one reason or another. Other times girls will try to get away with faking it, which is never acceptable, in my opinion. The bottom line is that the risk for me to pay a lot of money and not really get what I was hoping for is very high and that has changed my approach over the years in two ways:
1) When I started chatting, even though I always had my favourite models I kept returning to until they became unavailable, I still used to keep trying new models on the side, hoping to find someone better. A lot of the excitement for me came from trying new girls and the quest to find my next new favourite model. I used to look forward to every chat with a new girl. Now that I've found a girl who does awesome shows for me and I've had disappointing chats in the past with a lot of other girls, the bar is set a lot higher and my expectations for new girls are a lot lower, so the result is that it's much harder for me to get excited about new girls and I'm a lot more cautious when it comes to trying someone new. If my favourite model were to quit, I would find the prospect of having to find a new girl to chat with more of a headache than something to be excited about. For that reason, I have a much greater appreciation for her and I chat with her almost exclusively.
2) Aside from fetish chats, I recently started to do short "vanilla" chats where I just ask a model to show off her body and pose for me in my favourite poses. These are fun, far less expensive than a fetish chat and much less risky, because almost every girl will do it well. I still prefer to do this as a custom video rather than a cam show, but if a cam girl doesn't offer videos and she's uniquely hot, I sometimes give in and do it as a chat. This allows me to still chat with other girls and add a bit of variety to my chats without having to do a fetish chat.
Also, I think some Colombian models spoiled me by wanting to interact on Whatsapp, but now I enjoy that ability to just chat and share photos every now and then, as opposed to dealing with girls who want to charge me for everything. (Although I once had a chat with two girls and I asked them to kiss each other and they wanted me to pay extra - which was completely fair because that wasn't part of the original deal - and it was worth it for the hilarity that ensued with several aborted attempts and lots of giggling. It was really cute.)
As far as availability... When I started chatting most of the girls were either from the U.S., Eastern Europe, South Africa or the Philippines. While I like all types of looks and I have chatted with girls from many different countries, overall my preference is for Hispanic girls and until a few years after I started chatting there simply weren't that many of them. Now I often search for and chat with girls from Colombia, so the changing demographics and the fact that there are more girls to choose from has definitely changed who I chat with and it's not because my tastes have changed.
As far as my reasons for no longer chatting with certain girls... If it's a girl I chatted with frequently, >85% of the time it's because she disappeared and about 10% of the cases I felt like she started to take me for granted and stopped putting in as much effort into our chats as before. As for the remaining <5%, a couple of them told me they didn't want to do the types of shows we used to do anymore and another couple raised their prices where they wanted to get paid more than what other equally good girls I knew wanted for the same type of show, so I stopped chatting with them.