# Why is the specific frequency of globular clusters in cD galaxies so high?

I understand that cD galaxies, very large and bright galaxies, have more globular clusters than other galaxies. For example, by calculating the specific frequency for a cD galaxy, the number is approximately 18.6 in contrast to a M31-like galaxy with 0.73. What I do not understand is why the amount of globular clusters is so high in the cD galaxy. I do not know if a possible reason is that the number of globular clusters increases with luminosity and as cD galaxies have high surface brightness in central regions, the formation of globular clusters is more effective.