There are two main reasons why this happened.
1. Your are unable to connect to the localhost. This can be fixed by tweaking your xcode info.plist Exception Domain. See image below:
2. If you have a history of using VPN or if you are using VPN then you have to turn on your VPN in order to connect to the react native bundler.
In other words, this is a network issue and not a react native issue so make sure to double check your network settings. In my case, since I am using VPN and my MAC networking settings is configured to use the VPN so I had to turn on the VPN in order for the iOS simulator to work.
Hope this helps
