There was a guy working on the 7.7, but I believe he cracked his screen during a move. In addition, the 7.7 and 7.0 Plus have a major hurdle to overcome with the fact that the wifi driver is dual-licensed GPL/BSD by Atheros, giving Samsung an excuse to withhold sources for the wifi driver and wpa_supplicant changes.
As to I9300 getting support before older devices - Unlocked bootloader + ICS kernel source code + many of the internals being similar to the I9100/N7000 (GPU is almost identical, so is the MFC/FIMC interface, so is the RIL interface) meant it was one of the easier device bringups.
In general, as long as there are manufacturers who provide either fully unlocked bootloaders (Samsung) or lenient unlocking policies (Sony, also LG I think), you will see developers tend to avoid manufacturers with aggressive locking policies (HTC, Motorola). Even if someone figures out how to work around the locks, it's extra hassle that volunteers don't want to waste their time on, and many of us, on principle, refuse to support a manufacturer that continues with such an anti-developer stance.
I used to have HTC devices and loved them - but I will never purchase another one of their devices again as long as they keep up with the bootloader locking BS. Even their official "unlock" is only a partial unlock that leaves a pile of limitations which Samsung users/developers just don't have to ever deal with. My time is valuable, and I also have strong feelings regarding Tivoization.