Because intergalactic space contains very little gas or clouds to provide a medium for refracting light. Because of its high frequency, light refraction requires either a gravitational field a la general relativity, or a very dense medium ( glass or a thick atmosphere like the Earth's) and you do not find such media in intergalactic space. When astronomers measured the bending of light near the Sun at visible light frequencies, the presence of the solar plasma was not important, and was essentially transparent. At radio frequencies, however, the solar corona does refract radio waves significantly, and this effect was modeled-out and removed from the data, leaving behind a confirmation of the general relativistic effect.