Does anyone know why the rules state a a few specific calibers that are leagal for Field Pistol in addition to straight wall pistol cartridges that ar 1.29” in length or shorter?

I think the purpose was to allow the virtually straight wall calibers like 32-20, 25-20, 270 Ren, etc. This allowed equipment parity with NRA events. Eventually, the 22 Hornet was added.
The 30 Carbine is straight walled, 32-20 appears to have slight taper (I don't shoot the 30-20), but the latter is rimmed and ejects better from a TC. The added calibers allowed more of those quirky barrels to be used and by NRA guys that shot in both associations.

I would have expected that the too-long 270 Ren, being a straight Hornet, would have been added at the same time as the Hornet.

Interesting that pistol length (1.3") or shorter cartridges with a bottle-neck that pre-dated IHMSA and were available at the time were not added, like 221 Jet and 256 Win.

Two or three years ago I submitted a change to the cartridges for FP but it fell into a black hole and was never heard of again. Submitting rule changes these past few years is pretty iffy unless you happen to be one of the more well known in IHMSA. I suspect too that some cartridges were not allowed to prevent excess target damage and/or splash back since FP is shot at shorter ranges. A TC barrel with a 30 Luger or 30 Mauser would be a hoot but they are not allowed either. Even 7.62 Tokarev would be great for FP.