I think I agree with everything everyone's said - it's all of those reasons, and more
I really regret the passing of Latin-teaching in schools. That gave me a big gee-up in (a very basic) understanding of French, Spanish and Italian. Learning Dutch (admittedly by sitting in a bar and listening - oh, the price one has to pay! - and discovering that it's VERY similar to English but pronounced differently) gave me a good understanding of German. Knowing a little French enabled me to discover that Catalan is nearer to that language than it is to Spanish (and, let's face it, there are an awful lot of Brit tourists who'd be better off learning Catalan rather than Castilian Spanish). But that basic understanding isn't speaking da lingo.
As usual, there's more to it. It's all those funny conditional word endings!!!!! English is the one and only European language which doesn't have them - and that's why English (well, simple English) is just about the easiest language to learn. You can get away with murder when speaking English - it's not so easy to be understood if you mangle most other languages. English used to have those endings, though. The language which came over with the Angles (and Saxons, if they ever really existed) was as complex as any other when it came to case endings, and if that had been the end of the matter then we'd still be using them. But we also got invaded by the Danes. They spoke a very similar language to the "Anglo Saxons" - they came from the same place, almost. All of the roots for verbs and nouns were the same. The only differences were those endings. So, as the Danes and the Anglo Saxons had to come to a method of living together (and therefore trading, intermarrying, etc.) the endings were simply dropped over time. Hence standard verb and noun forms in English, with very few exceptions - no huge irregularities, no apparently logicless special cases to learn. Not so any other European language.
That makes English unique - certainly in Europe. Top that with the British Empire at its height and you have a lingua franca for the world - which is exactly what English is well on the way to becoming. It is the most widespread language and its use is increasing faster than any other - it is, eventually, going to become the science-fiction Terran language.
I don't think (I hope not, anyway) that English will ever totally supplant every other language, but I see no problem there - previously mentioned daughter was completely at home in English and Dutch, knew when either language was appropriate, and switched effortlessly. But it will become (it's just about there already) the first or second language of every human being on this planet a) because it's easy to learn in its basic form and b) because in it's fuller form it is recognised as one of the most adaptable languages around.
So, if ever you feel like shouting at the archetypical tourist Brit, if ever you become frustrated by our apparent ignorance of all things linguistic, just bear in mind one thing - it's all Denmark's fault.
Mike