Well, because colour-film deteriorates differently. Colours "fade" away or shift. Separating the colours from a good negative onto black and white film eliminates that. B&W film is very stable. Like "good for 80-200 years". And at any given time you want the colour composite back (nee the original print), you scan the separated film strips, usally YCM, realign them digitally, undo shrinking/expansion and there you have a perfectly coloured film.
That's why, for example, FujiFilm can win an Academy Award for perfecting a film product (Eterna RDS, 2011) - in an age where everybody is talking "film is dead". And in case you are surprised that even films that were acquired digitally are copied onto good ol' analogue film for archival, just read the insightful article on how to save Star Wars, Or find out why "storing it all on harddrives" is still not an option for the large archival scale (Library of Congress) in Ken Weissman's explanation.