I believe I recall that the brigade commander {Phifer] was known as a stickler for military precision, maybe even to the point of being labeled a martinet by some of the wild western dismounted cavalry troopers in his brigade.

Perhaps he saw the opportunity to create a full regiment of specially trained troops for the corner stone of his brigade rather than just a 3-6 company sharpshooter battalion as originally envisioned.

Since Williamson's battalion was being broken up at the same time, maybe he saw the chance to build a full regiment of elite troops, even at the expense of taking one company each away from the 6th and 9th Texas dismounted cavalry regiments. The more recent order of designating a regiment of sharpshooters gave him this opportunity.