Because of the cost of doing so. There are apparently 3 advantages to polygonal rifling:
- Higher velocities due to reduced friction of the bullet in the barrel, as the polygonal rifling has less surface area than the lands and grooves of a traditionally rifled barrel
- Less bullet deformation, resulting in reduced drag on the bullet which helps to increase range and accuracy
- Increased barrel life and reduced buildup of copper or lead within the barrel
The advantages don't make that much sense, I imagine, to adopt the expensive forging process required for polygonal rifling. If you want a bit more velocity, buy +P ammo. Nobody except LEO's care much about bullet deformation, and the barrel can be replaced under the lifetime warranty, if that ever became necessary.
In addition, polygonal rifled barrels, such as in Glocks, are very suseptible to excessive leading. In fact, Glock's manual warns of this, and also warns of the slide not returning to proper position, as well as barrel constriction, if the gun isn't cleaned and all leading removed very fasidiously.
All in all, it doesn't sound like that big of a deal, at least to me, that Taurus went the direction they did. They keep their guns affordable.
