More mass is available closer to the star and less far away because the disk density decreases with distance from the star. The temperature also decreases with distance, which can cause ice to form, increasing the solid mass available for planet formation. Additionally, the strength of the radiation and the intensity of the stellar wind also decrease with distance, which can also affect the amount of mass available to build planets.
However, the mass available for planet formation also depends on the protoplanetary disk's age and composition and the planets' presence, which can affect the distribution of the disk's mass.