It’s harder to review. As a reviewer it’s difficult to know which code change is related to which task.
It’s harder to verify. Did you really test every change you made?
You might end up with a “hostage” situation. There might be a few code changes in the PR that looks good and is really wanted, but other code changes in the same PR of lower quality. As a reviewer, should you just let these lower quality code changes slide so you can bring in the code change you really want? Probably not, but you’re going to let it slide either way.
Stuffing multiple tasks into one PR is often bad.