Hopefully this isn't a really stupid question but here goes...

The 10mm is only .0001mm larger in diameter than the 40SW. If that is the case, why did Sig use the P320 45ACP platform for this cartridge?

Cartridge pressure is close to the 40SW.
Diameter is very close to 40SW.

The only thing I can think of is the OAL.

Thanks.

You are correct with OAL being the issue/difference.

9 mm, .40 S&W, .357 SIG are all shorter.

10 mm Auto and .45 ACP are longer, requiring greater front-to-rear magazine internal length, and larger front-to-rear magazine well length in the grip frame to accept the larger mags.

Thank you - that makes sense.