I think that it's rarely performed for two (maybe more) reasons mikey. Opera fans (of which I am one) seem to expect Rossini's operas to be comic,Of course he did write "serious" operas,such as this,and the wonderful Ermione.Add to this the fact that this is quite long,and the tenor part is set quite high,and you see some of the problems with staging it.
According to an anecdote, when an admirer told the composer that he had heard his opera the previous night, Rossini replied "What? The whole of it?"