The reason, of course, is that so many were woven to satisfy the demands created by fashion in Europe and America from about the 17th century onward. There are a number of clues that we could look for. One is the pattern of wear. A piece that had been used in prayer for many years would probably show much more wear at the lower end than at the upper, since the user got onto it at the lower end and rarely touched the top. This would be the result of the user putting all of his weight at that level when kneeling.
Lets look at an example.
It is likely a commissioned work woven for Moslems, not for the Western marketplace. Some prayer rugs are of much higher quality in terms of fineness of weave, use of expensive materials like silk, and workmanship than most others of nominally the same type. For these reasons, it would seem more likely that the pieces were commissioned rather than being part of the usual output woven speculatively. Shirvan prayer rug, dated 1864. The Shirvan prayer rug above is probably such a piece.