Is there some kind of reason why people so rarely use Dragonstone Watchtower ?

On paper it looks great since you can easily get 2-3 power from them during game and even more importantly force opponents into awkward moments beetween giving your rush deck more power or playing key card like Orphan or Burning.

I bet it just doesn't fit into most decks... while useful, it probably isn't reliable enough to help Bara rush, and a delayed Knights rush might not be able to make room for it either. I can say from experience that it works great in an Asshai deck though.