Looking at my own, large collection of classic figs, I felt I only had a few double-based squads that I’d want to use (small and distinct enough) and those all had three figs each. Could have made it three at a time and you could replace the ones you lose, but having a hard limit was a simpler rule to explain and follow. Also, once we designed the Master HSC rules, a limit on the number of kings felt more thematically consistent.
And while classic checkers has no limits on kings, how often do you have more than three kings in a game? Probably doesn’t happen much, so probably doesn’t effect gameplay much to have a limit.
Please let me know how it goes if you do try it out!