I agree with the OP that it is actually no surprise, my feelings from my own experience are that a lot of fuss is made immediately after the birth then just as hormones come crashing down, most people disappear, by this I mean paternity leave ends, family return to their lives and you are left.
The isolation compounds everything else that's going on, a huge desperate love for your baby, wanting the world to be perfect for them, knowing it's not going to be, reliving experiences from your own life, re-examining other relationships. The whole world is turmoil.
For me, I got prescribed anti-depressants pretty quickly but this wasn't what I needed. A lovely HV sat with me, let me talk, listened, let me cry, didn't try to make things "right" but enabled me let my emotions out. It took one talk with her and I felt able to move on.
More time from people who understand and less medication, that would be my prescription. Let new parents talk and work through what they are going through.