I'd say the event system is the main reason. Not having to worry about integration or initialization of various components, HTML5 or not, is the main selling point to me, and I know how to do those things in HTML5/JS, Flash, etc. You're overselling your decision to use HTML5 as a basis for C2. HTML5 was a nice bonus, because I know what's possible with it, and C2 has some shortcomings compared to what the technology is capable of. Until the rewrite, those shortcomings will also effect C3.
Yeah, I never wanted to make games that you'd run in a browser. My decision to use C2 was due to the eventsheeting system. I didn't care whether it was html5 or not. All my projects are wrapped with NW.js because I just want to make a game you run and play like usual. I'd prefer if I didn't have to use NW.js
And yeah, I have become disappointed with certain shortcomings due to the html5 tech. So the html5 is definitely not the reason I'm using Construct.
I guess it is the reason Scirra is developing Construct, and that is fine.