Because it doesn't matter. The closed source tool isn't going to hide stuff in your text file. And should they change completely in a way you don't like or stop updating you switch tools you aren't locked in since its a text editor.
And honestly even with open source if the main devs stop updating or doing something you don't like you're time is almost certainly better spent doing what you're doing on a different editor or the old version then taking over development on yet another text editor.
I think you could also ask the question in a different way.
What is the market for closed-source text editors? We have open-source text editors, and freeware(?) closed-source ones.
So who would buy a closed-source text editor?
(Talking about a text editor here, not an IDE)
So then why not just open-source it? Seems to me there are only drawbacks in keeping it closed.
I've paid for textmate and sublime text because they were better than their immediate competition for what I use them for. I know lots of other people who have paid for one or the other. I have a really nice tmux + vim workspace I use a lot but for unfamiliar codebases I find exploring a lot nicer in sublime.
And honestly even with open source if the main devs stop updating or doing something you don't like you're time is almost certainly better spent doing what you're doing on a different editor or the old version then taking over development on yet another text editor.