Cameras often have wavelength filters to allow recording only certain wave lengths, that can be switched. So taking a colour photo requires three different exposures. Depending on how long you can take a single picture and how much changes in between you may not really get a useful result. A Bayer filter like in common digital cameras these days would also reduce resolution and restrict you at the sensor level to colour photos.

About the only use for colour photos in space is PR with non-scientists anyway. That was one major obstacle the Hubble Space Telescope had to face. It's a useful component, to be sure, but not one to justify adding much more weight or cost to a planned mission in all cases.

In case of Rosetta/Philae the probe was very weight-restricted, taking even a very long course towards the comet to save fuel since it didn't have much.