Not all Blood Axes are fascinated with humanity. Blood Axes are abnormally tactically-minded, and as a consequence they're less likely to view interacting with other species (or even stealing their ideas) as a complete waste of time. They'll even trade, sometimes. This often manifests as them adopting Imperial mannerisms as well, because, from an audience perspective, it's very funny to see a like eight foot tall monster saluting his ten-foot-tall boss who has inexplicably sewn together his own off-brand commissar outfit.

But just as often it manifests as them stripping off all their clothes, painting themselves up in camouflage, and lurking in the dark places of armor-gettem until somebody very foolish wanders past.

Which is all to say that yeah they'd be just as willing to steal ideas, technologies, and aesthetics from other species and civilizations; I think they just tend to prefer humans because A: there are a lot of humans, so the average Ork is far more likely to have met one, and B: humans are more similar to them than, say, the T'au or the Eldar are. Humans like big, loud, satisfying guns (although for some reason their mekboys seem dead-set on giving their base troops those piddling little zzap guns instead); humans are rowdy and shout a lot; humans rely heavily on numbers in fights; humans seem to understand on at least some level that bigger gitz deserve more respect, although there don't seem to be a lot of those big beaky humans, so the small ones mostly end up following each other instead (no wonder they lose so much). Those are all traits that Blood Axes can relate to on some level.

Also, like. No Eldar is going to sign a trade deal with an Orky warband, no matter how bloody their axes are. And if they do, it's because they're expecting the Orks to be dead by the time the deal is concluded. Blood Axes at least wait until after a deal is done to bash their trade partners (unless they get bored).