Well I'm not sure exactly why, but that's not the way they've been doing it which is why people assume they are going to continue their trend. In all honesty there isn't any reason why they couldn't but most people are arguing reasons why they wouldn't. The way they did it with Oblivion was having a countdown on their website, so it gave people a lot of time to spread the news (just look at how that Eurogamer article spread), but it still wasn't the official announcement, more like a announcement for an announcement lol. So it gave people the opportunity to speculate and spread the news to the non-hardcoe so everybody was watching and speculating before the announcement.
In any case they are still probably going to do all options, it's just the order that were trying to figure out. I don't think it's really all that important, if the non-hardcoe misses it on their website and blog they'll catch it during the convention, or the GI cover. Does the order really matter that much?
I understand the rationale behind it, I just don't personally agree with it. The gaming landscape - announcement plans, marketing, release schedules - has changed drastically since Oblivion. Oblivion was a (planned) launch title for an all-new generation of hardware, in a space where there was literally no real competition. Bethesda could afford to announce the game, wait a year, then show it off at E3 '05. Today, you've got a situation where, if you want to announce any game that isn't a sequel to last year's hit, you've got to hit the ground running or you'll lose the attention of anyone who didn't already care about it. Even an awesome series like The Elder Scrolls isn't impervious to this phenomenon, especially considering in the five years since the last game, a whole new group of gamers (read: potential customers) has now joined the gaming space. That means a whole new group of people that never played an Elder Scrolls game and don't know why they should care.
Like I said: there are other ways for Bethesda to hit big with an announcement. I just think, with the VGAs right around the corner, it's too likely a possibility to dismiss. If I were in their shoes, I'd have at least considered it. Especially since it's the last real publicized gaming event until PAX East and E3 - nobody announces at PAX because it's largely for marketing existing titles, and E3, well, we already know what Bethesda thinks about trying to compete at an event like that.