People are fixated on common usage because it’s common, and therefore, by definition, most likely to be unambiguously understood by the largest number of speakers.
The rest of this is in the spirit of modern linguistic nerdiness:
If there is a common word, it should be preferred over uncommon words simply for ease of communication. It is much more common in the English speaking world to say “a tour bus” for a bus that goes around a city near the sights to be seen, and while “a touristic bus” might be a perfectly acceptable synonym, it is less common.
The same holds for “salubrious”. While by dictionary standards it might be the best option, it isn’t that common, and most people would say “healthiness” or “wholesomeness” for salubridad and “sanitariness” or “healthfulness” for sanidad.
Source: USian immigrant to Spain married to a filología inglesa / translator
Another reason that English speakers talk about common usage is the ridiculous number of words in the language:
The RAE contains something like 93k words, including all the americanismos.
The Oxford English Dictionary contains roughly 470k words, and estimates that only 170k of those are in common current usage. So there are VASTLY more words in the English dictionary than most English speakers have ever even heard, much less could use properly. I didn’t know that the word touristic existed in English until I i moved to Spain, for instance.
So for English speakers, getting down to the 100k or so most used words means ignoring 80% of our dictionary. So when we say something isn’t common usage we really mean something between “no one has used that word in 60 years” and “I had to go look up if that even WAS an English word”.