To understand the different types of rifling you have to understand the different ways gun barrels are made.
Conventionally, barrels start as a long steel cylinder that's drilled (using a gun drill
- drilling long straight holes is tricky in itself), and then the rifling is formed by pulling a single point cutter (several times for each groove), a rifling broach, or a carbide rifling button through the bore.
That gives you your conventional land-and-groove style rifling. The equipment can be purchased for a couple thousand dollars or even made from scratch in a simple shop.
In more recent years, hammer forged barrels have become popular. These start with a short fat cylinder of steel which is hammered around a mandrel which is essentially a negative of the shape of the inside of the barrel. It's necessary for these to form polygonal rifling due to the way the steel is formed around the mandrel and the necessity of being able to pull out and reuse the mandrel when the process is complete.
Hammer forging machines can produce barrels quickly and cheaply, but the machines themselves cost millions of dollars.
Polygonal rifling has many advantages over conventional rifling in terms of bore friction, gas sealing, and minimizing bullets distortion; but it's not necessarily ideal for all barrels when issues of the scale of manufacture and absolute accuracy come into play.