You want it out, but it's pretty clear other people don't. Why are you able to force your personal views on other?

You are free to choose not to use Uber, others are free to choose for themselves.

Because that's the primary way that governments aren't just corporations: if the rulers (the majority, in a democracy) believe that Uber needs to go out, or at least needs to stop participating in the market in certain ways, the government can enforce that via its monopoly on violence.

This has historically worked pretty well for human society; for instance, if someone gets the clever idea to participate in a market by impersonating an existing, trusted market participant, or offering counterfeit goods, or using slave labor to plant cotton, or whatever other bad idea that is short-term profitable, the government can step in and say that this isn't long-term good for society.

Whether Portland is correct to stop Uber is a legitimate matter of debate, but that Portland legitimately has the ability to choose to stop Uber is, I'd hope, uncontroversial.

Why should a majority be able to decide who and who doesn't get to participate in the market?

Not "a majority," "the government". It happens to be the case that the government in the US is (in theory, and usually in practice) majority rule, but in a monarchy, the ruler has the unilateral right to decide who gets to participate in the market or not.

If you don't think this is legitimate, two questions:

1. Is it legitimate for the government to restrict foreign participation in markets, or to place rules like tariffs on their participation?

2. Was it inappropriate for the greater military force in the US in the 1860s to decide that Southern plantation owners could not participate in the cotton market if they continued to use slave labor?