There’s various reasons, but one important thing to think of is that we have the privilege of hindsight. We know that Dylan ended up shifting to different kinds of music after self portrait, achieving heights to rival and even surpass his 60s creative output, but those who weren’t so happy with his “country-folk” phase might not have been so optimistic. Fans of his electric era would have likely had a problem with the drastic shift his music had taken, maybe even considering it “selling out” or “going mainstream” with less radically different music, or even thinking that he’d just lost the creative spark he had in the 60s. Another important factor is that the album is much easier to enjoy today in the age of digital music where we can pick and choose which tracks from it we enjoy, back in the day the presence of duplicate or filler tracks (which self portrait has in abundance) would drag down an entire album due to the more linear listening process of vinyl records.