Baby has very little exposure to sunlight. For example: if you live in a far northern latitude, if you live in an urban area where tall buildings and pollution block sunlight, if baby is always completely covered and kept out of the sun, if baby is always inside during the day, or if you always apply high-SPF sunscreen.
Both mother and baby have darker skin and thus require more sun exposure to generate an adequate amount of vitamin D. Again, this is a "not enough sunlight" issue - the darker your skin pigmentation, the greater the amount of sun exposure needed. There is not much information available on how much more sunlight is needed if you have medium or darker toned skin. See the section below regarding amount of sunlight needed.
Mother is deficient in vitamin D - this is rare in western countries. The amount of vitamin D in breastmilk depends upon mom's vitamin D status. If baby gets enough sunlight, mom's deficiency is unlikely to be a problem for baby.
Take care your baby