Two reasons.

One, in my experience people with slightly above average political knowledge tend to be more partisan. This could easily be a selection bias, partisans being more likely to learn a little, but I think in general whenever people learn they are more likely to use it to amplify their confidence in their current ideology.

Two, a little knowledge can create undue confidence. There are a lot of issues people (rightly) stay completely out of because they have zero knowledge. If everyone had just enough knowledge to feel engaged by an issue the decision makers might become pressured to follow the partially informed judgments of the majority.