BowTech,
Great question.... I had someone ask the same question about a polar bear rug a couple of months ago. I like to explain it a little to each person so it makes more sense.
Most taxidermists tan, or have, their hides/furs tanned. That is why you want to do research on a taxidermist before you take your trophy to them...because a few either did or still 'cure' their hides. Curing is NOT tanning. Cured hides fall apart, no matter what you do, over a period of 4-10 years. Tanned hides will last a lifetime,if taken care of, however after about 6-7 years, depending on the type of tan that was done they can no longer be re-hydrated for purposes of a re-mount. The hair is set to the skin, however due to age AND the PH properties within the skin, activating it with water will damage and skin and hair. Hope that makes sense...
The other issue with a request to make a rug into a mount is that most rugs are cut differently than what is preferred for a mount. Also, during the rugging process, most of the time, there is small bits of the hide that are trimmed to give it a uniform appearance, thus making the overall mount smaller and harder to sew uniformly together.
Hope this made sense and pardon my long winded response. Joel- BRT