The 10 round limit for SSP for example was to accommodate the Californians.
Not at first.
IDPA was founded in 1996 with the AWB 1994-2004 in full swing.
The 10 round SSP and ESP limits were to conform with federal law affecting the whole country.
You might say that the 10 round limit was kept in place after the AWB expired in 2004 to accomodate California... and Canada, and maybe some other places that had their own capacity restrictions with a "level playing field."
Or that it was kept in place to maintain the rules, procedures, and records of 8 years of operation.