Afaik it's all based on a specific genre of "british colonial troops getting owned" art that was really popular back in the day, where they're all lined up surrounded by (imperialism noises) getting their asses kicked so the folks back home can see how brave and daring these basically prisoners of poverty conscripts were out there murdering people for the glory of English 4pm tea. So the 40k "giant line of dudes kicking ass and/or dying" is that mixed with hair metal, which is why everyone is always lined up line WWI on acid instead of, like, tactics.

Anyway, I much prefer the old, bright, super colorful, very silly art. WH40k is only fun when it's weird and goofy, otherwise it's just (fascism noises)