Why do we forget good practice?

I spent a lovely English Spring day visiting the Roman Villa of Chedworth. I came away marvelled by the way in which they had underfloor heating and an hygienic toilet system fifteen hundred (and more) years ago. When the Romans left our small island, those who remained ignored / forgot about (not sure which) these and many other useful inventions.  Why did they do this?

And what other well evidenced business related techniques and products have we forgotten about and are ignoring?