Think of it like this: Would the Sunwell have been given a lot of attention by players if it released with The Burning Crusade? No, because no one would be able to do it for a while. First, you had to get to 70. Then, you had to get through Kara, Gruul, Mag, SSC, and TK. Then, MH and BT. Finally, you were there...it would take months of work.
What raids and such we have right now for Wrath is gearing us up for the next content patch. The only time in TBC that Blizzard gave us a raid that anyone could start out with was Zul'Aman, and even then, you needed to be Kara-geared.

I'm assuming that Ulduar will be a harder raid than the raids we have now, and this is a way to get people geared up for it beforehand.
Just because you have something doesn't mean you have to use it right away.

Besides, why release a content patch that is made just for Lv. 80 stuff when you don't have people at Lv. 80?

WOTLK was new-content-complete quite a while ago, but it was still in beta for several months. Just because art assets, level designs, quest designs, etc. are done doesn't mean that all the bugs are fixed.

Actually I disagree. For raids in particular I think it's better to stagger their release, so we don't get what happened with BC (2.1 is the Black Temple patch!... So, uh, we're done with new raids this year, pretty much. Check back in April for the Sunwell!)

Which meant the Sunwell had to be tuned for people who had farmed Hyjal and BT to death. Not that I'm complaining about that too much, but still, staggering it like this makes a lot of sense to me.

We aren't talking the sunwell here, we are talking the FIRST patch. And it's done. There is no reason for it to not be included in the release.