Because, generally speaking, open source means no money/hobby development.
Listen--I respect your view, but I think you're wrong. I think many users are fair. I think most users are...lazy. And cheap. If there's an easy download that's free, they'll do it. If there is no easy download that's free, but an easy way to pay ,then many will take it (the iTunes effect). I think that there are many, many developers out there who don't care about intellectual property despite their avocation. An open source version would siphon a ton of sales, even if there's no downloadable binary (we are developers after all, we can get it built). And a downloadable pirated binary would become available anyway; heck, it will be available even as things stand now. There'll be a ton of demand.
I will say that developers are probably more sensitive to annoying DRM than others; I could imagine (hypothetically) some sort of draconian copy protection that would do more harm than good. But a guy that's offering completely unrestricted betas does not exactly strike me as the guy to do something like that, so I'd give it long odds. (By the way, there's an easy mark for the people who won't pay--just grab one of the unrestricted betas! Works great!).
Perhaps most importantly: it's an issue of control. Jon, thank goodness, seems to be taking extreme care in how his creation is being crafted. Have you seen Notepad++ lately? It's good for what it is, and has tons of features, but it doesn't have people raving about it like TextMate (and now, Sublime Text). The way it goes right now, Jon has every bit of control over his interpretation of what a text editor ought to be like. Happily, I rather agree with him (and apparently most of you do, too).
I have every bit of confidence that should Jon not wish to make money from ST any more, he would open source it. No, I don't know him. He doesn't know me. I'm just not that concerned, though. I respect those who are concerned, but I just can't bring myself to tell someone to give away their work. You want to invest all that time and make it open source? Great. Just don't insist that others do the work and release it on your terms.