Wow way to ask a simple question... haha :smile:
Here are the reasons why I think it is struggling:
1) Time Limit: This is the biggest reason why it is not succeeding like people thought it would, the time limit. People are learning to slow play the deck when they see a Gothirita (sp?) turned over. They just take their lead to time. In Top Cut, it takes too long for the first game to conclude, leaving a second game to go to time and a sudden death match. This is not a good situation for Goth.
2) Speed Kills: ZPST and tyRam are both too fast for most Goth players. If you are Goth you are relying on some lucky draws to get those basics evolved before they are KOd.
3) Vileplume (with Beartic, Yanmega, Mew, etc.) gets the better of this match up and tends to have a better match up against the field in general.
4) There are just enough big hitting decks to keep Goth down. Decks with RDL, Bad Boar, Magnezone, etc really feast on Goth. As long as they see a decent amount of play, Goth will struggle to consistently dominate the tables.