Why did apple have a 30-pin dock connector if USB can do everything?
https://www.reddit.com/r/apple/comments/yxrfu/comment/c5zw18d/
Why does Apple have a 30 pin connector anyway?

USB is a stack of protocols which is over complicated. Many USB implementations are riddled with bugs and people are used to just "disconnect and...

reddit

@SwiftOnSecurity I think the real reason is that in 2003 USB 2.0 was juuust starting to appear, and USB 1.0 transfer speeds were abysmal.

Moving iPods to USB would have been a huge performance regression for Mac users, keeping it on FireWire would have made Windows adoption impossible, and including both ports would have been ugly.