This is an interesting quandary of science at the time, and an unfortunate use of a push by special interests.
Coming into the '97 legislative session, elk were still a game species and under the purview of TPWD. About this time elk were starting to become popular in the Hill Country for game ranches, and they didn't like being told what to do with their elk, soooo, let's get them classified as "exotics". That puts them under the Department of Agriculture instead, and you get more flexibility to move them around. But also gets them technically reclassified as "livestock", and why Texas elk can't be book listed.
How TPWD went with elk were only historically found in the Guadlupes, I don't know. I was taught, and have always taught that elk were found throughout most of Texas until you get into deep East Texas. So keep in mind this would be consistent with the Rocky Mountain elk that up until "recently" (ie spread of white man) was mostly a plains species. I work with an archeologist quite a bit, and there have been several bits and pieces of elk found in Dallas County. I doubt that came from the Guadlupes.
Now we has to deal with the issue of the Merriam elk. People either really believe in them or they don't.....genetically, they aren't different from what is found in the southern Rockies. May be simply a matter of you have pockets of great nutrition, genetics and age that allowed for a large number of larger individuals. For a long time, a LOT of credence was given to the 1905 Bailey survey (awesome to read about). But you have to keep in mind this was a MAJOR leap by modern standards, not even really in place 20 years ago (aka 1997). Bailey didn't actually find any elk in the Guadalupes in 1905. He looked at a SINGLE skull from an old bull from just across the border in NM from habitat that "looked" likes the Guadlupes. Hell, most of us could walk it in a short time, so not much of a stretch. But that is effectively how we got to Merriams being in the Guadalupe Mountains.....for consistency purposes. In the past I have talked on here about the Merriams being In the Transpecos, but have adjusted my view on the matter in light of more/better information coming out the last few years.
Enter the desert bighorn. No doubt they were in West Texas, and between hunting and disease, they went the way of the dodo bird. So let's reintroduce a species that is eaten by everything, and seemingly susceptible to everything.....AND comes with a VERY WELL FUNDED cheerleading crowd. So how convenient that the elk got reclassified statewide. TPWD had the mandate to repopulate bighorns, and the elk were doing much better to the point of POTENTIALLY out compete the bighorns. I want to point out, the reintroduced desert bighorns aren't strictly the ones that were found in the Transpecos either.....but they have a very vocal special interest (not saying this is a bad things).
Keep in mind, when things don't make sense on the surface, there's usually way more at play than biology. And keep in mind, genetics have come a LONG way since 2007, and lots of information has changed. Now whether TPWD and special interest groups will change due to new info is an interesting quandary.