The real reason is that they don't want people showing that other designs work better and destroying the longbow myth.
You have two statements there.
1. Other designs work better.
Yes, in a modern context, but not for killing armoured foe at 200 yards. This is why the longbow evolved into the Victorian style target bow and the modern laminate longbow. Ultimately it was redesigned by Clarence N Hickman becoming more rectangular in section, the American longbow.
http://www.archeryhalloffame.com/Hickman.html He then went on to design the modern take down target recurve. He did that because he has lost some fingers in a rocketry accident and couldn't pull the necessary draw weights of the time. He needed a more efficient bow to enable him to compete.
2. There is no myth that the longbow / Warbow performed it's task admirably. The only slight myth is that it was all about the bow... it was just as much about poor French organisation.
It is ironic that in my youth most of the "how to make a 'longbow' " literature I could read was of American origin and based on the American version of the English longbow using woods like Osage that no timber merchant in the UK had ever heard of!
The rise of the internet and forums like this has ensured that the huge variety of bow types are there to be enjoyed.
To show I have no Longbow bias, I usually shoot a Yew Neolithic style bow
Del