There are some differences in features on the board. The SKR Pro 1.2 uses an ARM M4 processor, where the 1.4 Turbo uses an ARM M3 processor. There are some additional instructions native to the M4 CPU for some things that we don’t care about, but what we (might) care about is improved floating-point performance. (Do you use G02/G03 commands in your GCode?)
The official BTT site is a mess to navigate to do much meaningful comparison, with the specs listed in different orders, some comparison specs are simply unavailable. The difference for “turbo” seems to be a 120MHz clock, instead of a 100MHz clock, but the Pro (seems to?) out-perform it from what I remember.
As to choosing one to have “official support”, I’d guess that it’s basically that Ryan looked at the options, made an educated guess as to what was going to be around for long to be worth supporting that had a certain desired feature set, picking up that ball and running with it. The SKR Pro supports some really nice drivers, and some economical ones, has a good 32bit processor for “future proofing” and support for the 12864LCD as well as the touchscreen. (Bonus, you can use the touchscreen as a 12864, so good with legacy firmware code.) But that’s the one that he’s configured the firmware for, supports and sells. There’s 1001 different Marlin based control boards out there, it could be a full time job just keeping up with them.
The community support for other boards isn’t bad. It’s actually pretty good, I’ve found, but sometimes you’ve got to wait for “the guy” to respond if you go off the beaten path. Chances are that the latest greatest board out there is usable, but most of us aren’t going to go out and buy one just to configure firmware on it, when we have a perfectly good solution bolted to the side of the machine.