Throughout history, salt has been a dietary staple, a highly valued trade good, and a necessity for long term storage of certain food items. On the other hand, the world at large only came in contact with black pepper much more recently, initially as a luxury item. How did this spice, not earth-shattering compared to others with a similar history, become so common as to establish an intimate pairing with salt?
There is some initial aspects of the question that assume incorrectly that pepper wasn't valuable until recently. Black peppercorns are native to South and Southeast Asia. Most importantly to the spread across the world, they were grown heavily on the Malabar Coast in Present day India. During Roman and English times, this coast was a major section of the trade routes with "the west". There it was known as "black gold" and highly valued for it's taste, but it also has some anti microbial and rodent deterrent properties (albeit small).
The Romans brought pepper from Egypt north, and then the English (during their occupation of India) spread it to the parts of Europe Rome hadn't made it to.
It was traded in the aristocracy, for both it preservation qualities and because food gets boring, especially when most was preserved the same ways (though pickling or Salt) and this offered some verity. Also, having pepper in your food was a sign of wealth and prestige. Like many things adopted by the aristocracy, when prices declined to a point people could afford them, they gained popularity.
I hope that kind of clears things up.
sources: Innes Miller, The Spice Trade, this other reddit thread https://www.reddit.com/r/history/comments/16c6xm/why_did_ancient_people_value_pepper_and_other/ , "Antimicrobial agents from plants: antibacterial activity of plant volatile oils". Journal of Applied Microbiology