It’s rooted in how Bush talks, his speaking style infers he is dumb, ergo the foundation of BDS. Happens to a lot of southerners. Nothing could be farther from the truth, he beats the donks like a drum — WHICH JUST DRIVES THEM TOTALLY NUTS.
Lyndon Johnson, Jimmy Carter and Bill Clinton spoke with distinct southern accents and I don't recall anyone accusing them of being dumb.
Dwight Eisenhower spoke fitfully. His style has been described as convoluted and disjointed. It even has been said that his speaking style was to a degree cultivated. I don't known if anyone accused Ike of being dumb, however.