Because Twitter will definitely be here in 20 years and #s will somehow be less annoying then.
Somewhere out there there's a 30-something named PetRock Johnson who feels really bad for this kid. Not only is Hashtag an objectively terrible name for sounding like an old man's sneeze, it's also already outdated and mostly used by the most annoying people on Twitter at this point. The only silver lining will probably be that other kids will have to find out from their parents exactly why they should make fun of this child. At least we already know what she'll change her name to if she becomes a porn star: Pound.