The problem is that email is store-and-forward, and there's no standard for MUAs to independently interact and perform key exchanges via PGP or anything, so end-to-end encrypted email will always be a much worse experience than end-to-end encrypted IM.
Also, the heterogeneous MUA world, and the fact that users expect to be able to search their email even if encrypted, just makes end-to-end encrypted email a really tough proposition.
I could see something like OTR+PGP for email that could work, but the MUAs would have to get updates, and MUAs are "a solved problem". There's just no real work ongoing on MUAs.
Also, the heterogeneous MUA world, and the fact that users expect to be able to search their email even if encrypted, just makes end-to-end encrypted email a really tough proposition.
I could see something like OTR+PGP for email that could work, but the MUAs would have to get updates, and MUAs are "a solved problem". There's just no real work ongoing on MUAs.