OH! You musn’t use detergent on waterproof horse blankets!! Detergents are made of little molecules that grab water and the residue of it destroys the waterproofing. The best product for machine washing waterproof horse blankets is Nikwax Tech Wash, a non-detergent cleaner. You can also use pure Ivory soap, a scrub brush, and a high pressure hose. 
Tide is fine for stable blankets, dress sheets, etc, that are not waterproof.
Tide has ethanol in it–I guess that’s what gets most of the dirt and sweat out of clothes. Brands like Purex use sodium laureth sulfate, like what’s used in shampoo.
I use Clorox Green Works, $14.00 for 90 oz at Target. I only need a third of a capful for a large load and it gets all the dirt and sweat out and smells better than Tide.
I used Purex once. I had to wash the clothes twice with a capful each time and they still smelled like sweat.
While that may be true, don’t assume just because both things are made by the same people that they are the same product.
In addition to what others have said, you partially answered your own question. The very fact that it’s popular will drive the price up. (Supply and Demand and all that)
My apologies if the impression was I assumed it. It’s what a local Costco manager told me.
I don’t have any waterproof horsewear, but I’ll pass that along to a couple folks I know who have waterproof horse clothing. Thanks!