The reasons are mainly historical. English was spread first by the British through their empire and more recently by the US through its cultural and economic hegemony. It is now the default language when two foreigners meet in many parts of the world, even in the parts of Europe where either German or French used to fulfil this role. In many countries it is an official language simply because there are so many local ones!
I do not think the perceived "simplicity" of English has anything to do with it. Many languages that fulfil an auxiliary function are not necessarily perceived as "simple" e.g. Latin throughout Europe in the Middle Ages (and later), Russian throughout the former Soviet Union and Arabic as the language of the Koran throughout the Moslem world.
But it is a misconception to think of any one language as easier than another. It depends on who is learning the language, how similar it is to their own and how much they are exposed to it. For many people in the world bilingualism is the norm and they do not think of any one language they speak as being "simpler" than the other. For English speakers a language can be perceived as being "difficult" if you have to learn dozens of different forms of verbs or worry about gender. At a certain level a language like French is "easy" because a lot of vocabulary is shared. German, although more closely related to English, presents more "difficulty" because words are not so easily recognisable, even though they may have common Germanic roots. Also, if you are brought up in a country where English is an auxiliary language you will be surrounded by it and absorb its "difficulties" unconsciously. It also seems to be a rule that the more "simple" a language is i.e. the less you have to learn verb and noun forms and the like, the more elusive it is the more you get to know it. It is often said that the "easiest" widely spoken language to learn is Malay, but is is also said that it takes ten weeks to learn to speak it well and ten years to speak it properly.