Swift is designed to:

* be safer and reduce bugs * be more concise to increase productivity * be easier for the compiler to optimize * introduce functional programming constructs to make algorithm development easier

Objective-C is weak in all of these areas.

There is this nature of software tools to grow in backwards-compatible ways inevitably becoming evolutionary dead-ends. Apple tried hard with features such as Automated Reference Counting (ARC) but inevitably it is still C glued with a hard-to-optimize object system.