Why is katakana used for the onomatopoeia/sounds in the background?
Using onomatopoeia to stress sound effects in literature is not manga-specific or even comic-specific. For example, 1966's Batman TV series or even Nickelodeon's The Fairly OddParents had readily available audio to match their onscreen text with emphasis. But several cultures have their own origins. Tracing back when this particular artistic/literary device caught on would be pretty exhaustive, if even credible at all.