Harry was not allowed in Hogsmeade because he couldn't get his permission letter for Hogsmeade signed by his uncle. After blowing up his aunt out of anger, Harry had to urgently leave Privet Drive and had very little time to think about this. However, the actual reason for Harry not being allowed to go to Hogsmeade is believed to be the possibility of the Ministry and the Hogwarts staff keeping him safe from Sirius Black. Everybody believes that Sirius Black betrayed Harry's parents and was after Harry. However, this proved to be false in the end.