It's a decision made by NEMA when the entire concept of standard NEMA sizes arose so that if a US equipment mfr sent machinery to Canada, or a Canadian mfr sent something to the US, the starter sizes remained the same regardless of the motor voltage. NEMA was an outgrowth of the old JIC, Joint Industries Council, which was mainly the Automotive industry, who had plants on both sides of the border. So if GM moved a production line from Flint MI to Windsor OT, they might have to change out motors, but the starters or later, MCCs (after 1956) would all be the same regardless.
JIC/NEMA came up with the concept of standardized starters in the 20s when the automotive industry exploded. Automotive production lines went through major model changes about every 3-5 years initially, major meaning complete production machinery changes, not just minor tweaks of trim, grills or interiors. So to avoid complete shutdowns, the new production line was being built behind the old one while the old one was still in use which meant duplicating things like electrical control equipment. So to control costs, they came up with the standardized starter sizing so that line electricians could swap out and re-use starter buckets without having to have an Engineer make the decisions as to how they might be used; electricians were cheap and necessary for day to day operations, EEs were expensive and hard to keep busy enough to afford having them on the payroll between equipment changes. A NEMA starter could take the worst abuse an electrician could throw at it, based simply on the motor HP size. So a 50HP motor could be on a stamping press, a fan or a chain conveyor, the NEMA size 3 starter was fine no matter what. Expensive for the fan control maybe, but it was more expensive for Ford or GM to have a bunch of EEs waiting around in non-production change years to make that decision than to just have an over built starter for that fan.
IEC starters are the exact opposite, someone is supposed to "engineer" each application to pick the exactly correct contactor for the exactly correct application, duty cycle and expected life, so as to avoid buying even one extra ounce of silver in the contacts. That concept came out of Germany after WWII when our Marshal Plan money paid to educate thousands of EEs, who had no jobs when they got done. So in the 50s and 60s, German companies could get EEs cheaper than trained tradesmen (most of which ended up killed in the war), so they had them over think every little detail and "rationalize" every decision about which contactor to use, because materials like silver were extremely scarce and very very expensive for them. I worked for a German company in the early 80s, Klockner Moeller, most of my bosses were from that era. We had "Planning Manuals" that dictated the extremely tedious process of going through all the intricate details that went into picking out a freaking contactor, it was RIDICULOUS for a guy like me that started in a steel mill as an electrician using NEMA contactors. years later everyone started buying IEC contactors because they were cheaper and smaller, so hardly anyone sells NEMA starters any more, but trust me, they are NOT built the same.
