Why a chrome extension and not a desktop app? Because a desktop app is 3 desktop apps if you want to be cross platform, and since it is secure communication software, all of those edge cases that pop up in cross platform desktop application development really matter.
I think it's better that Signal takes its time and gets it right so that eventually we have a good solution. If Signal were to throw caution to the wind and hackathon up some desktop apps, then we may never have a single good option.
Signals competitors don't share it's security standards, and so it's not really reasonable to compare it to its competitors feature for feature. I expect that adding a given feature or other unit of complexity bears a higher cost for Signal than, say Whatsapp or Telegram. In the meantime, we still have the Signal mobile apps for situations where inconvenience isn't an insurmountable barrier.
[1] Signal-Browser doesn't seem to be able to add contacts properly when used with the production server, and the staging server looks like it's down right now.
[2] NW.js currently refuses to recognize Signal-Browser as a Chrome App unless I rename package.json to something else. Remote debugging doesn't seem to work with Chrome Apps running under NW.js at the moment - the inspector just gives me an empty response for each page I try to access. And there will need to be some way of configuring the Chromium engine to use Signal's self-signed SSL cert, though they'll have to solve this for the Chrome App as well.
edit: Apparently alpha versions of NW.js now support running Chrome Apps. This could be interesting. https://groups.google.com/forum/#!msg/nwjs-general/YuwMHd_uv...