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Different materials for different applications and considerations.
Possibly the first consideration is material strength. The strength of yarns, and subsequently the textiles they're woven into dependends on fiber length, or the length of the "staple" as it's termed by weavers. Linen has a longer "staple" than cotton, which is the shortest of the natural fibers. Hemp has the longest staple. Synthetics (like polyester) have an infinitely long staple.
In my experience (which depends in part on observations made in the high 'n' dry mountain west, where humidity is always very low) cotton canvas stretched taut and optimally prepared will "slack off" under the attack of working a painting - vigorous brushwork, scraping, etc. Under the same conditions, linen remains stable and taut. Linen on a stretcher is more readily affected by changes in ambient humidity, however, and can slack off noticeably in very damp weather. Using linen too light in weight, too loosely woven, or too thin in thread count exacerbates the problem if lighter materials are out of scale for the size of a piece.
The long-term durability of linen depends on proper sizing and priming. It's a "nature" thing; oil of the flax seed (linseed oil) is particularly effective at rotting the stem of the plant (flax, or linen fibers). If it's not properly sized, linen canvas rots more readily than other natural fibers, but all become brittle and decay in time if allowed to absorb any vehicles and oils from priming and painting. Polyester doesn't decay in the presence of linseed oil, and can be painted on directly, without sizing or priming, if one wishes to do so.
The cachet which linen enjoys has less to do with being the best textile for painting supports, and more to do with the fact that it was the commonly used material of the "Old Masters". I've noted many patrons and gallerists will "ooh and "aaah" over linen, as if it were the gold standard, so there remains some advantages of salability for using it.
Of the natural fibers, hemp is likely the best of the three, as it is strongest, resists slacking off under a hard work-out, and seems to be least affected by changes in ambient conditions. It is, however, a bit more difficult to stretch and size.
Trivia: the word "canvas" is a corruption of the Latin word "cannabis", the name for hemp. Roman tri-remes had hemp-canvas sails.
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