Because a gravitational redshift of the magnitude seen in distant quasars would require that the masses of distant galaxies be vastly greater than typical nearby galaxies that otherwise look identical to the distant 'high redshift' ones. This leads to an implausible and inconsistent picture of the universe, in which galaxies can look the same, but have vastly different masses. Independent indicators of the dynamics of distant galaxies show that their masses are entirely in line with those of nearby galaxies, so there cannot be enough mass within distant galaxies to produce a gravitational redshift. This leaves the cosmological redshift as the only reasonable explanation that has survived repeated tests.