I think there is a number of issues here,
Firstly Malta has alot of international air force bases stationed nearby that these aircraft can easily get to the show.
I get the impression the show is at an airport which would help no end !
Also every airshow period here, you can hear people on the radio saying warplanes shoudln't fly over N.Ireland,agree or not this is someting the orgnaisers have to be wary of.
I agree that for the likes of the Italians, Greeks and French, Malta is a short hop away but that wouldn't apply to the Germans, Dutch and Belgians. The RAF aircraft probably come from their base on Cyprus or Gibraltar. The Yanks have bases in Italy and Germany.
I think the show is held at Valetta airport which I'm assuming is shut down during the show.
A question that could be asked as far as the Portrush show goes, and one which TTF might be able to answer, is who are we actually asking to come? Is it the same groups each year or do we rotate them? During RIAT the B-52 that was on display took off during the Sunday show to go do a fly-by at a show in Spain!!
How difficult would it be to get an F-15 to come over do a short display and then fly back?
I realise that cost is usually the deciding the factor for any aircraft that attend but if we don't ask then we don't get.
Would it be viable to spend the money on fewer displays, but which are more high profile, then spend it on multiple aircraft that people are not fussed about? The Red Arrows are the classic case, people come to see them and then leave after they've finished.