Apple likes to control the whole product as much as possible. The iOS/OSX ecosystem is absolutely big enough to support an exclusive language (see Objective-C)

Apple likes to control the whole product as much as possible. The iOS/OSX ecosystem is absolutely big enough to support an exclusive language (see Objective-C), and Apple chose to create a new language that matched their goals instead of adapt something they don't control and isn't ideal. Makes perfect sense, and Swift was by far the most impactful announcement at WWDC.