Because the gravitational effects of dark matter can be seen more locally.

For example, the rotational velocities of stars within spinning galaxies don't match the gravitational force that should be generated by the visible matter within those galaxies. To produce those velocities there must be a lot of unseen mass within those galaxies. Mass that is beyond the visible universe wouldn't increase the centripetal force within those galaxies.

Similarly we need unseen mass to account for the amount of curvature that we observe in instances of gravitational lensing. Such mass would need to be between us and the light sources, not beyond those light sources.