Using published information, you can generate a rough/general time line
30-30 Winchester - designed and marketed in 1895 for the 1894 lever action
30 Remington - introduced 1906 for the Model 8 autoloader. More or less the rimless version of the .30-30 and first called the .30-30 Remington ... now considered obsolete
219 Zipper - 1937 by Winchester, a 22 cal version of the 30-30
219 Donaldson Wasp - 1937, shortly after the 219 Zipper was introduced. More or less a shorter, improved Zipper
6mm/30-30 - wildcat first documented in the mid 1940s
225 Winchester - similar to the 22/30-30, shortened and smaller rim. 1964 vintage
30 Herrett - 30-30 shortened and improved, 1972
270 JDJ - 6.8 version of the 225 Winchester case, improved, could be seen as the original precursor to the 6.8 SPC (a rimmed version). Don't tell JD I said that, though! 1978
30 American - special version of 30-30 brass with small primer and small flash hole for match grade versions of the 30-30 wildcats. 1986
There are others such as the 7 x 40 based on the .30 Remington that were used in competition.
So there is not really much new under the sun. While indeed one could go out and "develop", "design" or "invent" a new version of a wildcat based on the 6.8 case, we felt that it would be easier, faster, cheaper to use existing tooling. If a customer can take an existing .223 barrel, clean up the chamber with a 219 DW reamer and use existing 219 DW dies, to us, that seems to make sense. You can load the heavier bullets without having to seat them too deep, you get better performance than the .223, load data is available for starting loads ....
By no means am I marginalizing the work of the SPC team, Docs pics show they have been there, done that, gone over all the old versions. Just seems some of the writers out there have forgotten their history (just like one who said there was nothing in the US arsenal remotely similar to the FN 5.7 x 28. Seems the Hornet and 5.7 MMJ slipped his mind ...)
From my little corner of the world, it would seem that the 22 version would have made sense, as it would allow use of the existing projectiles, barrel blanks, flash hiders, suppressors, etc. Bolts and magazines are all that needed to be changed. 223 would not chamber in the 22 SPC and the other way around. The 22 SPC in a 10.5" should outperform the M855 and improve the effective distance (I was unable to duplicate the M855 velocities in the Ammo Oracle FAQ without going to very high pressure but this is likely a limitation of my software and not a reflection on the validity of the data)
Anyway, I dug up the 10.5" 22 SPC barrel and if someone is interested, I can see about firing some rounds of a chrony and seeing what it might do ....