Because of popularity or the received popularity.
FOr some time people kept saying "Pathfinder overtook D&D". Ans many believed it. Some believe it even today even if facts show that its wrong.
So they chose the game they thought was most popular PF1, but almost no one knows pathfinder compared to D&D. And then when 5e released and all the marketing made it seam like a huge success (again some people still believe this to this day even though 5e was the least successfull version) they changed to the new most popular game.
Its really not so hard. Content creators try to make content for the system they think are popular. Same with boardgames and other media. You can see that content creators for boardgames and computer games even make content about popular games they dont like.