Texas is different than the other border states in that there is no Federal Easement, the snaking Rio Grande is the border line and private landownership up to the River.
There are many landowners who’s land runs right up to the river, ranchers use the water for cattle, farmers use the water for irrigation and some landowners have inherited the land from ancestors that date back to the 1700’s.
There is also major recreation areas + Big Bend National Park. Two very large Lakes with resorts. And there is Apache Indian land.
In the other states, there was a Federal Easement so the logistics were much simpler.
Now that said, strategic fencing is more effective, less costly and can be done rather quickly with the backup of Guard/Border Patrol and equipment.
And you are right, the reason there is no Roosevelt Reservation is the presence of the river. But in fact there are some very effective fence segments, for example just east of El Paso, and further south toward the LRG Valley where the locals have collaborated with CBP to build combination levies/fences. These accommodate the irrigation needs. In my experience, some of the landowners who hold land that was part of an original Spanish-Mexican land grant are among the most insufferable, left-wing open-borders, Mexicans-with-a chip-on-their-shoulder America-haters I have ever come across.
Let's face it, in large parts of Texas such as the LRG Valley, Laredo, El Paso and other lovely, prosperous spots, Mexico has completely take over, colonized, imposed its political practices (?como se dice Patrón en engles?) and gone a long way toward destroying the Rule of Law. Yes Texas is different, and Texans are going to rue the day they did not stop the colonization before it was too late. Compromise is not always a virtue. Do you Anglos really think Mexicans are going to be as kind and accommodating and generous to you as you have been to them?
Ni modo.