Grand Unified Theories and Theories Of Everything are difficult to test and disprove. They cannot be tested on current or even reasonably imaginable future particle accelerators. We can check that a theory agrees with observations in the low-energy regime, and whether they match astronomical observations of faint relics from the universe's high-energy era.
Given these difficulties, selecting a good such theory becomes more of a question of aesthetics than science. Which such theories are beautiful and which are ugly? What defines beauty of a theory? Occam's Razor is likely important. Given a precise definition of beauty, find, perhaps computationally, the optimally beautiful theory which agrees with weak and faint observations.
The E8 Theory Of Everything likely attracted attention due to its aesthetics.