I have no hard evidence to back this up, but anecdotally I'm fairly sure Liverpool has a much higher international recognition and appeal than manchester, certainly in places like the USA and China. Doesn't Liverpool have the oldest Chinese population in Europe?
As someone said 4th most visited city, only surprise is its not 2nd or 3rd ...
The problem is not that Liverpool doesn't have the market it's just Manchester has the first mover advantage ...