Classes can reveal the properties of their members more fully (to understand the differences between calcium and magnesium, for example, you should know why they are both alkaline earths) but classes can also obscure them (the Pagans and the Germs were both American punk rock bands, but to me their songs sound nothing alike). Classes should be used with care everywhere; there’s probably no way to fully avoid them.
But you aren’t asking about classes in general; you are asking why poetry critics and reviewers seem to classify and classify, whereas fiction reviews try to avoid it. Perhaps it’s because few books of poetry can count on a buzz produced by their authors, or by a publicity campaign, or by grassroots, independent-bookstore-sales-driven chatter, all of which can justify (to assigning editors, to casual readers) space and time for extensive reviews of single volumes. Poetry reviewers, poetry critics, even very academic ones, need other pegs on which to hang their claims.
Novelists, necessarily, work in sustained solitude, when they are working (however gregarious they become otherwise), whereas poets can work in solitude in short bursts and then come together to discuss—and make programs and slogans about—what they made.
Poets also seem to attach themselves and their work more often either to their peer group, or to their teachers; some poets can tell you where and with whom they studied almost in the way that classical musicians can tell you about their teachers, and their teachers’ teachers. If novelists do that, I haven’t seen it.
For more, buy Steve’s book, Close Calls With Nonsense.