The technical issue is fairly well summed up by Sam James from Gentoo at https://gist.github.com/thesamesam/223949d5a074ebc3dce9ee78baad9e27
A rather good video by Theo discussing the human component of the exploit:
Soldiers dropping their swords. It is a highly effective technique!
Why wasn't it used then?
Probably because no one ever tried it, or else it was not recorded to history...