HAT
I was reading across some books for my recurrent study... And this question came to my mind just for curiosity....
What i've read on some books is that the 50' is set for several reasons...
Can be derive from the approach path 3º or any other angle for obstacle , eye reference , energy management...
But mainly it was how all the performance analisys calculations/design were made... Ie. All the landing performance computations are based on the aircraft overflying the TRSHLD at 50' . Sounds obvious to everybody , passing the tshld at 100feet and flying a 3ºGPA would most likely result in touchdown 900' beyond the the 1000'mark and subsequently an increase on total landing distance of 900'(270m).
That's what i've read and looks quite acceptable .