Because that has been the promise for at least the decade or so I've been reading it, now. See the FAQ, which has said very close to the same thing since I first looked, way back then.
Because LWN is a flagship FLOSS community publication, and as such, it should walk its own talk. Many of the projects and hardware manufacturer's drivers that LWN was covering a decade ago as closed source have come around and are either directly open source now, or they're cooperating with the community and open source alternatives are now available, yet LWN, one of the FLOSS community would-be flagships, is still stuck with the same now rather hollow reading promise it had way back then.
It wouldn't be so bad if LWN weren't the in the highly visible FLOSS community flagship publication position it's in. Honestly, as someone else mentioned the time has passed when the LWN code would be the valuable contribution it once could have been. But by now, LWN is badly undercutting its own FLOSS message and the efforts of many others in the community as well, exactly BECAUSE it's in the highly visible flagship position it occupies, yet the code remains closed despite a decade (or more) of promising to open it.
What has the community done with others who have promised that they'd open their code for a decade, yet it remains closed? What sort of community credibility do they have? Would the community have tolerated a hardware manufacturer promising sources for the kernel and GPL software they ship, and not delivering for over a decade, or would they have been on the receiving end of a law suit well before that, with LWN's blessing as it covered the news? Of course LWN doesn't have the legal obligation, but having made the promise and as the community spokes-site they are, they certainly have the moral obligation. Why does LWN continue to get away with it?
More selfishly, not that a few subscriptions make that much difference to LWN, but while I was a subscriber for awhile and at least initially I felt good about it even when I didn't have time to read LWN because it was a way I could contribute to furthering the community, eventually I got uncomfortable enough about the discord between the message LWN promoted and its actions, that I found I could no longer in good conscience subscribe. But I'm selfish and not being a subscriber is inconvenient. I'd love to be able to read LWN's feature articles as they're published, and I miss the reply notification feature even more, but were I to subscribe just for the convenience while LWN remains closed source, it'd setup a personal moral discordance that I'm not willing to live with, so I just live with the inconvenience. If LWN would just open up its code as it has promised, it would thus eliminate a big personal inconvenience, as I'd then be able to subscribe in good conscience again, and thus have access once again to these valuable LWN subscriber features that I sure miss!
But for now, I've resigned myself to not getting reply notifications, or being able to read feature articles before they're old news. And... I guess I can still hope that in Jon's final instructions (which hopefully won't be read for decades yet, but it happens to us all!) there's provision for finally opening the code, because at this point, I've about lost hope of that promise ever being fulfilled otherwise. Not because he doesn't want to, but simply because "in real life (tm)" if something's not made a priority, as this evidently hasn't been despite the promise, there's generally always something else more important to do first...