HTML5 is new and a lot more hyped. It makes sense, from a financial perspective, to make a tool to occupy a less crowded market, especially if you're developing it from scratch, as is the case with C2.
Additionally, from a developer perspective, it's a lot
easier to
- get funding for an HTML5 game
- get people to buy a HTML5 middleware if you're an influencer/affiliate/partner
- reach a wide audience
than with a traditional exe exporter.
Apparently, Scirra's business strategy is to empower indie developers and help them make money, unlike clickteam, for instance, which caters to hobbyists (all MMF devs who attempted to create games commercially seem to waste more time working around bugs and "newbie-friendliness" than coding actual games).
Even the SDK is a lot easier than what you'd find in Unity, Unreal, Source and other big platforms. If there's a feature you don't have, you can either wait for Ashley or work around it with a custom plugin - that speaks a lot about the maturity of the platform. Also, with each new *big* feature (groups/families/image points/collision masks) you get less and less reliant on Ashley to do stuff for you.
This is the reason scirra has my money.
Besides, no one ever said an EXE exporter was out of question. If an Exporter SDK existed, people would be working on EXE exporting already. Let the product mature a bit and get all the core features. Buyers were even considered "early adopters" until a couple weeks ago!