Popularity and advertisement of those are what makes them sell better. It's like how Avengers :Doomsday is trending now, selling Funko Pop of Dr Doom is booming.
Harry Potter Pocket Pop is one of the set I used to own as kid. Sadly, after moving I lost all of them. I've been looking forward to replacing them but couldn't.
Funko pop action figure t-shirts are not the real deal. This is why I don't focus on them because it's a waste of money. They will fade and wear out quickly but not the toys.