It was a one-stop shop that had all your content in an easy to read format sans ads and tracking. I think Google realized that if everyone went with Reader their cash cows of ads and tracking would disappear, so they killed it.

Its probably trivial to replicate, but RSS/Atom wasn't very popular to begin with and this move just motivated others to give up on it. Webmasters didn't want to have that weird RSS icon that no one understood and designers probably didn't like that their hard would went away with RSS.

On the flip-side ad blockers and things like Firefox's and Safari's reader mode help a lot. I think its not a big coincidence Chrome is late to the reader mode game either. Google wants you to load that ad tracking .js file because that's their bread and butter.