I feel like there needs to be some historical reasoning behind this. If we look at Western comics there's black and white comics, and comics done in color throughout it's history. During the US occupation the Japanese, and any future manga artists/writers, were probably exposed to colored comics as well.
Was there a financial reason for this initially and it became tradition? Or is there a different reason?
This can hardly be answered historically, but I will give you an answer anyway:
Japanese manga are published in magazines, printed on cheap paper and with a few coloured pages. As these are not collected and often thrown away after reading (the tankobon editions are kept), there is no need for durable paper. And so as to keep prices low (imagine a magazine with 200 to 1000 pages (phone book comparisons abound) for 3-7US$) coloured pages are an extra for popular series on a few pages.
I am pretty sure, that this is not a traditional thing. If by means of new technology, colour printing would suddenly become equally cheap as monochrome printing, the publishers would abandon monochrome printing in a heartbeat. The reason was and is to this day purely financial