Pocket was pushed pretty heavily and basically shoved down the throat of Firefox users. Many obviously complained about this behaviour, either because they had no use for Pocket or already had a different solution. Mozilla was basically mimicking Microsoft behaviour of just forcing products/features onto it's users.

Shortly after the Pocket launch Mozilla stopped pushing Pocket and it became less visible in the Firefox UI. Now it's just a tiny grey button most don't click. So you're either use Pocket and like it, or you don't even think about it.

The main complaint, as I remember it, was mostly how Mozilla positioned Pocket. Some people picked up Pocket over the years, many liked it. These are not necessarily the same people who objected to have Pocket thrown in their face.