Because Mozilla has made decisions which people disagree with (the mr. Robot thing, the new advertising thing, Pocket,...). Not saying 'Mozilla is terrible, everyone should switch', but it's understandable that some people may be through with some of these shenanigans.
This was such a mild thing that got so much pushback (meaning they won't do it again) that it makes me trust them more, not less. A disabled plug in that when enabled didn't really even do anything, compared to insert what any other browser does.
What is the issue with pocket? Sincerely interested