Sorry, I don't have enough familiarity with Kotlin to intelligently answer this question. As far as Swift goes, it has many strengths, some of which include:
- an emphasis on safety: stringently type checked code, optionals instead of null-pointer exceptions, and extensive syntax for working with optionals
- progressive disclosure: easy to get started with but featureful (you don't need to use every feature at first)
- protocol-oriented programming: a style that focuses on composition, giving protocols first-class status
- multiparadigm: can be programming in a procedural, object-oriented, functional, or protocol-oriented style
- modern syntax: this is especially true for people transitioning to Swift from Objective-C
- great learning environment in the form of Playgrounds in Xcode on the Mac and the Playgrounds app for iPad for younger kids
There are more but these are probably the main attractions for people learning the language (besides wanting to make iOS or Mac apps of course!).