If they are already at a disadvantage, this means they are not being challenged and learning as they should at home. Delaying their entry into school becuase they are not "ready" only delays their learning of the skills they need to enter school. There is a reason they are not ready, either economical, learning disability, other disabilty, ELL student, culturally, or access to materials, thus delaying them continues the delay in their learning that they are already experiencing.
Those who are disadvataged now have more access to a preschool education than before. However, once they leave preschool they may still not be "ready" for kindergarten by the standards of the school. There can be a multitude of reasons why a student may seem not ready for kindergarten but that does not mean that they should be delayed. They still need that challenge or else they will not grow and potential learning difficulties and other delays may come to