It's not easier to learn because of grammar or vocab, but simply due to network effects. If you want to learn it, whoever you are, and wherever you live and whatever your mother tongue is there is the highest possible chance on average that you can access people who speak English, find high quality reference materials, and find native media that interests you.
If you run those considerations through all possible language pairs it's easy to see that these are the main factors that influence difficulty. Grammar, vocab, and pronunciation are a distant second.