It probably helped that there was a lot of latent frustration about the Objective-C syntax, which looked very foreign for developers coming from other languages (apart from smalltalk :-)). So by announcing a language with a familiar and modern looking syntax, they captured the interest of a lot of developers who was attracted to building apps on iOS, but didn't want to use Objective-C.

I think that it also helped that they had a lot of high quality documentation (like the Swift book) ready from day one.