I wouldn't say that SMA is "the" standard RF coaxial connector, I would say that it is one standard among many. No connector has emerged as the best for all uses, which is why there are many standards. Other connectors such as the "UHF" (badly-named because its performance is terrible at UHF), BNC, F, and N connectors were invented earlier, and these that I mentioned are still in wide use.
I wasn't there when the SMA connector was invented, but I'll guess that the designers had in mind a small and inexpensive connector with a low insertion loss up to frequencies of a few gigahertz. The SMA has shortcomings for sure, such as not being able to handle lots of power, but the designers surely assumed that users needing better power handling, for instance, would just use a different connector. People have voted with their money and made the SMA connector a resounding success, just as other connectors have been successful for other niches.