Probably because they'd fail.
To my knowledge, there is only one design of free-float handguard currently being purchased in quantity by the US military, and it is a chunky beast compared to the slim and sexy rails released in the last few years.
I'd love one of the newer rails, but after reading long-term durability threads, watching vids like the above, and asking a similar question in regards to maintaining zero on a handguard mounted front BUIS, I've come to the sad conclusion that a lot (maybe even all) of the high-end light & sexy rails being sold still aren't ready for prime time. Granted, I've purchased items that I knew ahead of time might not be fully idiot proof, but it seems to me that we're putting a lot of eggs in one basket when it comes to extended FF rails, and I'm skeptical that many of them are really up to the job.
BUT I WOULD LOVE TO BE PROVEN WRONG.
What I'd personally like to see is a torture test where an extended FF setup is repeatedly dropped (5 or so minimum) onto a concrete driveway from multiple directions without significantly shifting the front BUIS zero as compared to a non-free float.
Give me that in a slim, light & sexy package, and you got my money. Give me that in a detachable format and I'll buy two.