To be fair, they are trending leftward. And not all of them are red; it kind of depends on what you define as suburbs. The inner suburbs that are near Cincinnati proper are probably Democratic-leaning at this point, if they weren't already. They're probably red (but trending leftward) becuase they are rich and white, historically a prime demographic for the GOP, and to a lesser extent, still one today (yes, they're trending leftward, but to attribute only them to the leftward shift in the Trump years is ignoring demographic diversification, which plays a big role in suburbs shifting leftward).
It's worth noting that those three counties were all in George Wallace's top five in the state. The ancestral conservatism of the region is very powerful, and they have yet to diversify or undergo major economic upheaval.
