In many cases, it’s a lack of cognitive flexibility, i.e. an inability to see a matter from more than one perspective. F. Scott Fitzgerald said that “The test of a first-rate intelligence is the ability to hold two opposed ideas in mind at the same time and still retain the ability to function”.
Someone without much intellectual firepower may base their worldview mostly on what resonates with them emotionally. If the basis for your worldview is emotional, than you won’t be able to open yourself to other perspectives. You will come up with (or, more likely, borrow) enough rational sounding arguments to make it sound like you came by your views through a reasoning process, but they’re really just there to lend some apparent substance to your emotion-driven positions. And because your emotions are strongly invested in the position, you’ll react emotionally toward opposing views. This leads to a situation of polarity.