Because the 9780 was on T-mobile and not on AT&T.
I Found this kinda lame.
Huh? The 9780 is exactly like the 9700 except for Tmobile. There is always a different version number for t-mobile because it runs on a different frequency for 3G even though it is GSM. The CDMA versions for Sprint/Verizon also have their own version number. So yes, the same phone did come out for AT&T. The 9900 is for AT&T, and T-Mobile will get a different version number.
THE 9780 AND 9700 ARE NOT THE SAME THING. There's a reason why the 9700 has been out for a much longer time than the 9780, and why tmobile carried both the 9700 and 9780.
the 9780 has a much better camera and twice the onboard memory. cmon now.