Most Hottub Covers you see on spas in showrooms all over the world were just an afterthought to the hottub. More effort was put into the development of the hottub and designing it to heat the water and pump it through the assorted jets. Understandably so. More money will for sure be spent on the Hottub than what goes on top of it.
Having the Hottub itself properly insulated is important but the nature of heat is to rise. Just throwing a piece of rigid foam building material over the top of the Hottub seems logical until you actually put it into use. What works fine in a dry environment in construction is not anything like the moist, warm steam saturated space above a hottub.
The air spaces in the rigid foam begin to fill with moisture and the foam becomes completely saturated. Long before you notice the added weight the insulation value of the foam hottub cover is greatly compromised. If you asked the foam manufacturer about it they would agree that this product (the foam) is not designed for a moist environment.
So why does it persist as the most common hottub cover? Too much money is being made in selling you replacements that will end up the same way, to change. It keeps you coming in to the hottub store at regular intervals, spending money to help them keep the doors open and to get you to look at their latest models.
Lets look at this in a totally different industry. The auto industry has been chugging along for years churning out basically the same models year after year with very little in the way of real innovation. Years ago in the 1970's there was a gas crisis. As a nation we went from huge gas guzzling muscle cars to more fuel efficient models. Fortunately for the consumer Japan had been thinking ahead and produce a number of smaller but economical cars. If Detroit had been doing the same thing, no foreign vehicles would have ever gotten a foot hold in the USA. But the US automotive industry is extremely reluctant to change and very narrow in focus.
I don't know what it will take for Detroit to change. Are there other alternatives out there? Most certainly, and they are selling like hot cakes.
In the Hottub industry it will most likely come down to something similar. Consumers will more and more often refuse to take home the foam Hottub Covers that the dealers try to throw in with the sale. Instead, they will tell the dealer, no thanks I'm getting a SpaCap Hottub Cover.