The White Sox were at the bottom of ratings last year in no small part because they're rebuilding, because they made the decision in 2015 and 2016 that they would sacrifice future years to win those years. There were games in ~2014 when the Astros had something like <1000 people tuning in, when they were playing on Sundays and the Texans season had started. Now the Astros are back to getting strong ratings across Houston, because they're a better team. Once Kopech got hurt, how many of us wanted to watch that last week? I tuned in for Hawk's last day.
There's reason to believe that viewership should recover to reasonable levels in this market...if the White Sox can turn around their franchise. The worry is that the White Sox have been bad for so long that at some point, the erosion of viewers and interest will become locked in and permanent, and even if they return to winning they'll only be in the middle of the league in ratings and still rivaling the Indians and Rays for ticket sales. I don't think we've gotten there yet, but if the first wave of talent in this rebuild flops, it'll have been >a decade since the White Sox's last winning season by the time the next wave of talent arrives. By then I could legitimately seeing there not being enough people left who care to support the franchise even if they get back to winning, and that's a situation where, when the stadium lease runs out, heading somewhere else makes sense.