They explain it here[1] on their discussion forum but still it doesn't make sense to me. If is a statement in every language I know and returns a value, but still those languages have ternary operator. I suspect they don't want to end up with a language similar to what we already have. They want to be different. Even when it isn't necessary.
Ternary operator is a C-ism. It's common in C, C++, C#, Java, PHP, Swift, Javascript, ActionScript, and related languages, but not present in most of the rest of the PL world. 'if' is an expression in Lisp, Haskell, ML, Scala, Ada, CoffeeScript, SQL etc. A modified form of the if-statement is an expression in Python 3 and Ruby. Go and Pascal have no related concept.
[1] https://kotlinlang.org/docs/reference/control-flow.html