A lot of people were using RSS. Then Google found RSS feeds were hurting Google ad revenues and so Google killed it. Since Firefox these days trails Google, it too stopped supporting RSS feeds
I was there during the turndown. As I remember, RSS had almost no impact on revenue positive or negative. That is why Reader was shutdown actually. Because not enough people used it to impact the bottom line and make it worth keeping.
Multiple factors go into the decision to kill a product; Google+ had a much larger investment and profile, it would make sense Google would be more skittish about killing it than Reader.
