I work for Siemens in the US and we make all 3 types of contactors; NEMA, IEC and Definite Purpose. To reiterate what others have been saying, I usually sum it up for people like this:
NEMA starters were/are designed to take the absolute worst thing you can throw at them and survive with a reasonable amount of serviceable life that will likely exceed the equipment that it is controlling. The reasoning behind it was that auto makers would typically change out a complete production line in no more than 4 years, usually around 3 (now it's 5 or sure, but times have changed). That meant that they had to build a completely new line in parallel with the old one prior to the switchover, so to save their capital investment dollars they demanded starters that could be easily re-used without a lot of engineering effort every time. Engineers were expensive, electricians were cheap. That ended up being the NEMA design we love today because you can confidently re-use a NEMA starter for anything else in your plant without getting an engineer involved.
IEC starters were designed during the pre-WWII heydays of European expansion when engineering was a trade rather than a profession. So in Germany and France and England (who joined IEC much later actually), there were people with engineering expertise on every street corner, needing work. In fact, many governments in Europe after WWI had labor laws that essentially mandated that companies employ more people than were really necessary. So to save money on components (especially when most of their raw materials were imported), they used all that available engineering brainpower to decide on the exact amount of contactor necessary for each and every application; no more. They would "rationalize" every installation and process whereby they knew how many operations it was likely to see, how many times per hour or minute, starting current, likely voltage drop, etc. etc. etc. ad nauseum. The end result was that to properly select a contactor, it might take an engineer 1-2 hours on each machine application. If you had a machine with 4 or 5 motors in different uses, that added up to be a lot more engineering time than anyone would ever spend in the US. IEC contactors were also selected based on the expected life of the machine; there was no intent to re-use them somewhere else because you would still have an engineer rationalize it all over again anyway, so the contactor cost was relatively irrelevant. The "throw away" concept crept in in the late 70s only when automated production lines made them too inexpensive to bother fixing, but it should be noted that only the smallest of contactors are considered too cheap to fix. All large IEC contactors have replaceable contacts (at least those from reputable sources). IEC contactors today are still designed with that engineering intensive mentality in mind, although the selection process is a lot more streamlined now.
Definite Purpose contactors are a US concept that somewhat mirrors the IEC mentality, but is reserved for high volume OEM applications and requires even more engineering time because you must investigate and justify every tiny detailed aspect on your own, including short circuit calculations, wear time vs warranty of your end product etc. etc. So if you are designing a machine once and will make it thousands of times without redesigning it, DP made sense because the engineering expense was amortized by volume. DP contactors are terrible choices for one-off applications however because the ratings on the labels are even more application specific than IEC ratings are, so it's really easy to misapply them and end up with damaged equipment or a fire.
