Most poems are basically less than 100 words. True, you can convey a brilliant massage with a few lines of creative words, but people spend less than a minute reading your poem.
I think it will be unfair for a piece with 100 words to earn more than a piece with 800 words. And that's how the algorithm sees it.
So that's why poetry pieces earn less. Maybe the poets can try writing longer pieces to match the length of prose contents.
Medium isn't just about encouraging talents but rather talents who capture readers engagement. That's why you see a heartfelt short story about 250 words go viral more than a well researched article of 1200 words.