When Cameron speaks, he has an air of authority about him. Whether you like that air or not (and it can be off-putting and smug), it's there and that's what other countries want to see.
Miliband just looks gormless, and has an incredibly patronising manner when replying to a question. He'll get rolled over on the international stage, so forget any deals with the EU to strengthen our position there.
Out of them all, Clegg is the one who has both the air and the down to earth chat. That's why a lot of people voted for him last time, based purely on the TV debates.