Why does Brave have a more compelling sell? What does it offer that could be sold to users as a feature they need? I wouldn't know how to explain Brave compellingly, unless I'm using some of the same arguments I use for Firefox (which are weakened in Brave's case due to using the same engine as Chrome).
On the other hand, explaining Firefox seems easy. People around me seem receptive to both privacy and closed garden concerns. I explain to them how their data is used if they do not take precautionary measures to prevent it. I also tell them that the web they know and love will get increasingly suffocated and rot away under the pressure of Google which has no concern for them or their needs. I tell them about Google's recent efforts to prevent ad-blockers from working well. I also mention other malicious tactics, such as AMP and the effort to hide the URL.
I then set up uBlock Origin (making a point it's the best ad-blocker but will stop working on Chrome shortly), Facebook Container and Multi-account Containers for a few of the most obnoxious websites. Multi-account Containers has a setting to make the association of a particular domain with a container permanent[+]. I give them a short introduction to containers and teach them to recognize when a website is open in a particular container (there's an indicator in the URL bar).
This works in my experience. It sticks. They continue using Firefox and at times even spread the idea to their social circle. It helps that in recent times concerns about Google, Chrome and computer surveillance are talked about in their regular news channels, so that I am not the only source where they hear about this.
> They care about... a smooth experience, I guess?
Ask them again. Try to understand their preference well enough that you can explain it clearly.
I am quite confident that they don't actually care about anything Chrome has to offer at all[@]. They're simply used to the colourful circle icon because they are being bombarded by it everywhere. It's the same as with IE before, only this time around it's a colourful circle instead of a blue 'e'.
> Chrome is amazing. Having most people switching away is a hard sell.
Firefox is amazing (in a technical sense) too. It's just as smooth as Chrome for me and sometimes even smoother since I'm a heavy tab user. I dislike some of Mozilla's decisions as well, but this takes a backseat to the more pressing issue of vendor lock-in given that there is no mainstream alternative.
[+] This could use a bit of UI work. After you assign a website to a container, you have to open the website in another container so that it asks you whether you always want to open that website in that container (it's a checkbox).
[@] As a disclaimer, I'm just extrapolating from my experience. Perhaps your parents are an exception and they do have a strong preference for a Chrome-only feature. If so, this is a useful thing to learn.