Domain reputation looks at who sent a message; IP reputation looks at where the message was sent from, i.e. the device or server corresponding to a specific IP address.
Providers want to protect customers from spammy or malicious content, and use IPs to determine the quality of a sending environment and make assumptions about the quality of a message, too.
Unlike your IP address, which you can change relatively easily, your domain sticks with you—which is why receivers are placing a lot more emphasis on domain reputation to determine your fate in the inbox.