Historically, it appears that closed source front-end editors like Textmate, Sublime Text and others find more success and usage in the market than open-source front-end editors.
I wouldn't classify Sublime Text as a front-end editor. When I think of front-end editors, I think of Coda, Espresso, and Brackets. Sublime Text is better compared to Notepad++, Notepad 2, Gedit, SciTE, Emacs, and Vim, which are all open source software and extremely popular.
If you're going to market your text editor as hackable and as flexible as Vim and Emacs [1], it's not a stretch for the developers interested in your product to assume it's going to be or should be open source software.
Fair enough. I use ST2 like Coda/Espresso/Brackets for my own front-end development b/c of the app's agility and abundance of useful plugins for the front-end.
Keeping Atom partially closed source, again pointing to historical marketability, could give it more credence among developers of both ends. There's a reason closed source, proprietary editors are so massively popular vs. open source editors which often can't seem to get meaningful market share (whether Enterprise or Sole Proprietor/Freelancer).
That said, I really wish an open source editor, on the front-end specifically, like Brackets or even Notepad++ would become something I want to use more than ST2, Coda, etc. but there's always something missing.
Atom has potential as a partially open source editor but it remains to be seen. I'll need to be drooling in order to truly make a switch.
I'd be very interested to see data that indicates that closed-source text editors have more market share than their open-source counterparts.
I'm quite positive on the other end of development, mainly Microsoft devs, they're almost always using proprietary software like Visual Studio. Sadly, Microsoft dominates the Enterprise and these environments are nearly half the job openings I come across/hear about.
I suspect one of the big detractors from Brackets is that it doesn't [yet] support VIM/other keymaps out of the box (https://trello.com/c/ckAel1Cl/358-118-keyboard-shortcut-prof... ). Additionally it could have set itself apart as a possible thick-client NodeJS debugger, but again not out of the box (http://blog.brackets.io/2013/08/28/theseus-javascript-debugg... ).
While developers are used to installing features they want, Brackets suffers from the fact that there isn't any built-in package manager and therefore these types of optional features are gated behind a Google search (where most of the top results are questions about support, not the answers).
Actually, Brackets has a built in extension manager. You can find a list of extensions here - https://brackets-registry.aboutweb.com/ - and in the product you can go to File->Extension Manager.
All the extensions in the registry can be installed from within Brackets.