While a mouse requires very little training and is the most common way to operate a desktop computer, they unfortunately require a flat, open surface and take up more room than alternative methods. Mice are easy to use and most users are familiar with how to operate one, but if a desk is not available many users will opt to use a trackpad instead.
Mice often require a cord that can get caught or unplugged from behind the computer, or the wireless options can lose connection and require batteries. All mice require some sort of flat surface, which is perfect for working at a desk but harder to use on a portable laptop. Mice are easy and consistent, but are not quite as versatile as a trackpad.