Because we care about what Milton wrote a lot more than we care about what some random book buyer wrote.

Why? We obviously read the notes and we didn't especially care for them before we found out they were Milton's, so why is that? This implies that we care more about a famous person writing dreck than an anonymous person writing fantastic insights.

The article mentions one allusion Milton made to Shakespeare. It is possible that we can learn more about what ideas Milton was referencing and responding to in Shakespeare. It gives us more insight into him than into Shakespeare.

Ah, that's fair.

If you find someone’s copy of a book that was an important influence on them, then by reading their marginal notes you can learn something about their work or life. We probably won’t learn much new about Shakespeare from reading Milton’s comments, but we can potentially learn a lot about Milton.

Perhaps not in this case, but often marginal notes are incomprehensible without context about what else the writer of the notes was reading, thinking about, or working on. Once you know who wrote the notes you can often understand more of their content and relevance.

That's part of it, I suspect. It's all so subjective, after all. And also, as noted, it aids interpretation of Milton's work.