I am sure not everyone agrees with me, but I believe Google single-handedly killed RSS by first creating Google Reader and using their power to make it ubiquitous and then killing it when there was no competition left.
Websites tend to not like to have RSS support due to it directing traffic away, and thus, denying ad revenue. It also allows easier aggregation from competitors/third-parties, which also denies ad revenue. Therefore, RSS is, at most, implemented as a short excerpt, with the intention to direct the user to visit the website.
But the number of sites, and the easy availability of readers for it (it was built into all of the major browsers at one point), have both decreased dramatically since the early naughts. If 10% of sites offer RSS feeds, I'd personally be quite surprised.