I started using Pocket half a decade ago.
By today's standards, that's a long time to use most non-social media apps. Yesterday though, the run ended as I switched to Instapaper and became a paying subscriber. I can't remember Pocket changing significantly at all over those last five years, other than being purchased by Mozilla. Perhaps that's the problem.
It felt old, and my perception wasn't being delighted anymore. Instapaper seemed really fresh. Because they had been in the news recently (albeit for mostly unrelated reasons), I decided to take another look.
When I started exploring Instapaper's features, all of them were clear improvements or additions over Pocket's feature set: Highlighting text, send to Kindle, no ads in the feed. I felt the same excitement about Instapaper that I did when I first started using Pocket. I get to carry around all these wonderful stories with me all the time.
Near the end, Pocket became an article graveyard. I wasn't even excited to open the app to read these wonderful stories anymore.
All that said, Pocket introduced me to the concept of read-it-later apps. (I never used Delicious.) And much had changed since I started using it. Today, the content I consume looks largely different along with my reading habits. I also have a Kindle. This combination of things could have been the main reason.
Most likely these things were just the tipping point though. Pocket didn't do anything majorly wrong to lose me as a user. But, they did not do enough to keep me either.