Linked from
The 14 pages that link to Chromatic aberration, each with the reason it gives.
Optical telescopeRelated: It is a characteristic limitation of refracting telescope objectives.
DispersionRelated: Wavelength-dependent refraction spreads colors out of focus in lenses.
Thin lensRelated: A single focal length cannot describe a real lens equally well for every color.
History of the telescopeRelated: It limited early refractors and helped motivate reflective designs.