Maybe for a couple reasons:
1) Apple has Aperture, which is in Lightroom's space. If Adobe waited another 6 months to get the beta out there, they would lose HUGE market share on the Mac. There's no such danger on the PC yet, as there's no Aperture equivalent there. Yay Apple!
2) I hear Lightroom wants to leverage some Vista technologies on the Windows side. Maybe not require Vista, but use some things that aren't in Windows just yet. As such, they're waiting for them (I'd have to guess it would be an XP service pack) before they throw a beta out there.
Nevertheless, the message is pretty clear: If you want to use the BEST photographic tools, you really have no choice but to use a Mac
