As someone who use to most play Poppy for 2 months after her update went live, I can honestly speak for myself. The reason I don't play her as much is simple: I like to play other champs sometimes. She's not a bad champ, not in the least. I'll gladly pick & play her if I felt like it. But there are plenty of other champs I would like to play, whether because of playstyle or character, that have thing Poppy can't provide. That's simply why I don't play her as much as I use to after she got updated.
She's a comfort pick for me, a champ I'm willing to play in most match ups & team comps. A champ I'm willing to go for a Juggernaut or full tank build with if the situation calls for it. For me, Poppy is fine. I honestly don't care why others aren't playing her.
Yeah, she was nerf quite a bit, both directly & indirectly, but I adapted. Just like I will adapt to her upcoming buffs next patch. For those who don't know, her passive shield now scales as she hit certain levels (15% at level 1, increased to 17.5% at level 7, increases to 20% at lvl 13) rather than just being 15% based on max health all game, her passive cooldown for early & mid game & the movespeed you get from her W has increased at all levels.