Assuming someone will ask, "Why do we talk about what is (law) rather than what could be (ideals)", you're perfectly fine to hold the ideal that political beliefs should not be discriminated against. But you probably aren't happy with just believing in an ideal, and so you want to know why political belief is not a legally protected class like age, religion, sex, race, etc. This means we have to include real world considerations, including legal history/precedent and the realistic implementation of law, when arguing whether political beliefs should get legal protections.

To make a quick initial argument, I think political belief is hard to protect because it is nearly indistinguishable from any kind of belief. At that point, how sensical is a legal system after it becomes impossible to discriminate on any kind of assertive opinion?

The line is already difficult and blurred with religion (which is protected because of centuries of prior belief and political situations), as we see in the case of a religious baker refusing to bake for gay customers. I'm not saying there's an easy or happy answer to this and similar disputes, just that protection of political beliefs would make things even worse without much benefit.

The other thing to consider is that as someone who has political beliefs and wants to assert them, you have the entire U.S. political system to participate and effect the change you want. But being political is not necessary when following a religion, and in some religions, it's outright antithetical. These folks would have little recourse to petition politicians and lawmakers.