US and Aussie tourists sometimes do multiple European countries in the same trip. Liverpool to Europe flights are popular with US tourists.If you're talking Chinese and US tourists, they aren't going to be flying to Manchester in huge numbers. They'll fly to Heathrow, "do" London* first, then use Avanti West Coast (or HS2) to get to their preferred bit of the North West. Meanwhile, European tourists wanting to visit Liverpool will just use sleasyJet or Eireflop to fly to Speke instead.
UK regional airports are more for British people flying other places than tourists coming in.
* And Oxford, Stratford etc.
The list of destinations which have direct flights from Liverpool changes all the time. At one point you could fly to Cologne and Berlin from Liverpool but couldn't from Manchester, now it's the other way around.