Why Wigglytuff isn’t on the defender tier list?

Her mediocre defense cancel the large stamina so she is not very bulky, though she can spam charge moves with the amount of damage she takes.

She used to be

Because she's not Blissey

Her low defence makes up for it. Wobbuffet isn't anywhere near the tier list either because despite the massive HP pool, it goes down fast due to how much damage it takes.

Additionally, Wigglytuff is screwed over by its dual typing. If it was just fairy, it might have some use, but taking neutral damage from Machamp really hurts in a meta where you really need to either be Blissey, Chansey or Snorlax or a bad matchup for Machamp.