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Answer by William Flanagan, longtime manga and anime translator:
Quite a few reasons.
First, price. The Japanese manga magazines are phone book–sized (Harry Potter–book sized for the younger set who’ve never seen a phone book) weekly magazines that do their best to allow even elementary schoolkids to buy them without breaking their allowances. So the magazines use very cheap recycled paper and only one color of ink. You get some 300-600 pages of manga for under $5. And although they have some advertisements, there are relatively few. Again, for the most popular magazines, they come out every week.
U.S. comics tend to run about 22 pages of artwork with quite a few advertisements every month. And those run $2-$5.
Second, distribution of work. Manga are essentially done by one person. That means for most manga, you have to draw and ink 30-40 pages of manga in a month all by yourself. (Although the most popular manga are weekly, most manga artists in the industry work for monthly magazines and don’t make enough to afford regular assistants.)
Any colored pages are also done by the artist alone. The artist also does sound effects themselves. The artists working on weekly manga may have assistants, but they are mainly for grunt work of filling in black areas, background details, and laying down tone. It’s the artist who usually still draws and inks the characters and also does the coloring for the rare colored pages. (There are reports of certain very popular manga artists who basically have their assistants do all their work for them. I’m sure there are examples out there, but as far as I can tell, this is not true for most of the manga artists.)
In comics, there is usually a separate writer, penciler, inker, letter (who not only does in-balloon lettering but also sound effects), and colorist. If the work of the other artists is done far enough ahead of time, the colorist has plenty of time to do his or her work.
Third, time before deadline. The system for U.S. comics starts the process for each comic months in advance, giving each person in the process enough time (theoretically) to complete his or her work.
Manga artists tend to work right up to the printing deadline. There are many stories of editors waiting on the artist’s couch, then rushing the finished pages directly to the printer.
Finally, it’s art. Have you ever seen a really well done black-and-white movie? If not, try it sometime. It conveys a certain mood, especially in the use of stark shadows, much better than color can. Manga, as an industry and art form, have built up plenty of excellent techniques for how to use black-and-white art, and in some cases (as exemplified by Marvel’s and Viz’s early experiments with colorizing manga in the late ‘80s and early ‘90s), the color actually lessens the impact of the artwork.
There are probably other reasons that I’m forgetting, but I think those are the most major reasons.