Given the target platforms Id hazard a guess its using SDL, hence no os3.x/68k version. While there is SDL for os3.x it typically increases requirements, so making it the realm of emulation only.
It using SDL is just an assumption on my part however. Could also be that theyre wanting a certain quality of graphics and sound, ergo using 16/24/32 bit color graphics + rta .
While 68k/aga fans are often familiar with the quality attainable on their machines, it can take some effort and knowledge to get good results when downscaling. Perhaps theyre not willing to compromise and/or spend the time required for the sake of 1 platform.
Who knows though, if they can receive a proportional amount (per platform) of "donations" from people under stipulation of a 68k/aga version then maybe they'd be willing to outsource the graphics and sound work required if they can find someone to volunteer their time.
Again, this is all guess work on my part, only they know for sure thus far, but if enough people interested contact them with a friendly inquiry then Im sure we could find out. :-)