Sodium TripolyPhosphate is the popular preferred alkaline builder in powder cleaning formulations of all types, because of it's "bang for the buck" value, and other desirable physical characteristics. So, why don't all manufacturers treat it as such? The bottom line is regulations, product certifications (e.g. GreanSeal), and how a particular manufacturer might choose to market the product for acceptance. Sodium Carbonate found most favor when US EPA identified phosphates as detrimental to the environment. As a result, several decades ago US EPA regulated phosphates out of existence in most consumer products. Hence, Sodium TripolyPhosphate was essentially taken away from consumers, but not so for industrial and commercial uses. It was reasoned that the reduction in environmental impact would be substantial enough if only consumer products were devoid of phosphates.
Manufacturers of powdered laundry detergents (because they are consumer products) had to "formulate" a "plan-B". Sodium Carbonate (AKA Soda Ash was a less expensive alternative builder, and it contains no phosphorus (Phosphates). Being less expensive, everything is OK, right? No, not actually. Soda Ash is harder to dissolve in cold water, isn't as chemically reactive as a builder as STPP, and has a few other negatives

. The laundry industry has been instrumental in creating textured Soda Ash to make it more free flowing with liquid additives, like surfactants, fragrances, cosolvents, etc. As well as making them easier to dissolve. But the greatest reason for its use in commercial and industrial products, I think, is the 50 state acceptability of the product. You see, even though Federally, it is just fine to use phosphates, some states have laws that are more strictly interpreted to make phosphates much more regulated than they are Federally. Hence, if you are functionally able to eliminate phosphates, you may have an advantage. Maybe not entirely today, but any day that a particular state gets a notion to crack down.
There is also another reason for Carpet Cleaning Manufacturers will use a phosphate-free builder like Soda Ash. It is for Green Seal compliance. However, as I will explain, it doesn't necessarily mean a level playing field. The GS-37 standard specifies that cleaning products for industrial or institutional us (all us guys),
as used shall not contain more than 0.5% by weight of total phosphorus. Notice I underlined and bolded the words "as used" in that last sentence. A small minority of , "less than mainstream" manufactures have exploited this as an apparent loophole. Certain GS-37 compliant products get around the rather low permissible phosphate level by specifying ridiculous usage ("as used") instructions of super light dilution that make the resultant usage mixture have the requisite low phosphate level. All the manufacturer must do is state in fine print, that when the product is used in
that particular manner, on "really light soiling", the product is GS-37 compliant. In big print, you are free to claim the product itself is GreenSeal approved. Never mind that only a handful of users will actually only use that incredibility light dilution, only once, when they find it doesn't work. Then they go ahead and mix it much more strongly.