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It’s cheaper. In Japan manga volumes are about 700 yen, which is about $5 currently. No budget for color printing there. Manga is often drawn by one person who is already massively overworked just to hit serialization deadlines, so no time (nor budget for assistants) for coloring.
If people want to learn more about all of that in a fun way, they should read the “Bakuman” manga by the authors of Death Note, the story of two mangaka and their journey to becoming authors for Jump magazine. I really enjoyed it myself.