I think it is less expensive because it is a newer product looking to expand into a niche market.
The original engineering for it was to act as a panel between dual pane windows to create a four layer window - basically three air spaces. The current max is two air spaces... and three would mean a very heavy window if made from glass.
No one ever got it to work with any durability in that format... but marketing came up with this concept... and lots of people have access to it in rolled form at this time.
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