The reason it hasn't come out to date is that it's Neil's least favorite record. I like it, though it's a mess. It's interesting as much as a psychodrama as for the music. Neil's doing too much drugs, he fires his friend Danny Whitten on the eve of a huge, high-profile tour, Danny then dies by his own hand, and Neil has to go out and face huge audiences expecting to hear Harvest. The record is an incredible document of those encounters.
As for the format in which it's being re-released, it's been noted since the '70's that those 4 records are "of a piece" - call it The Danny Crronicles (I know, I know, Bruce Berry too). It's the journey of a man going through hell, and emerging the other side.
As for why now, and in this format, it's for Record Store Day. it's good to support the survival of record stores.
Not exactly. He hasn't reissued it yet because of the way it was mastered. The recording and mastering were done straight to the CompuDisk (or what his producer David Briggs called the CompuFuck), so there is no way now for it to be remastered, remixed, anything. He has said that it would sound terrible on CD.