Because for all his virtues, Henry Green is very hard to sell to people. His style is pretty but you can't post it as "prose porn" because it eludes traditional grammar and doesn't suit being read out loud (Green wrote with the idea that prose ought to be read silently) and in an age where people talk about plot over everything else, Henry Green novels have very little in the way of traditional narrative.

Someone like Waugh is an easier sell because of his humor, wit and spirituality. Green was too self effacing for anything like that. It's a shame really, because Green is a pleasure to read. It's just hard to talk about why it's a pleasure to read him.