Answers to questions about "popularity" are always in marketing.
Let's start by thinking as a sales person from engineer's point of view. Who created Swift? Apple. What is Apple? It's an IT giant and #1 company in the world. When developer Bob created language X and posted it on his blog, let say, 50 people in his town read it. When Apple presents something - billion of people all around the world will know about it.
Another reason is in product itself, what that product was created for, how people can use it. Swift is a modern language and today it should be used to write any of Apple's software from MacOS till WatchOS. Again we see here a lot of platforms and almost everyone have Mac or any other device from Apple while Bob's language can be used only on much smaller market.
As you can see both marketing and the market of Swift is huge. That's why it is so popular.
I have used Swift and would tell that it is really simplier then Objective C and Swift 3 even simplier and better. Apple also tries to sell Swift for Linux development and it has resources to do that, so the popularity of Swift will keep growing.