While Led Zeppelin were not really metal, they were very popular and had very many heavy songs, and set many standards for the genre, such as highly distored guitars and wailing vocals.
But then bands started emerging that were more purely metal then others, such as Black Sabbath and Deep Purple. These bands became very successful and other bands started following in their wake, with the American Grand Funk Railroad and English Machine Head and Uriah Heep.
Taken from LiveVideo
Metal music struggled to sell during these times due to the punk rock movement, which was hugely popular, with alot of sales being taken by other more mainstream genres, such as disco or rock.