I think if I were Singer one thing I would be concerned about would be politicizing my philosophy too much. So if he solidly aligns too much with <obviously left-wing party social policy plank> he stands to alienate half or more than half of his readers. From his point of view, it may be better to only advocate for voluntary of your own free will charity and hope that people get the hint on the systemic change side of things. Also, considering this stuff came up in the 70s, there's probably a bit of calculation going on regarding that political climate. (Thatcher and Reagan were just around the corner so maybe that relates)