Because the general rule is Free speech if there isn't an exception. In Europe I would get charged, arrested, and/or fined for flying a Nazi flag at my house, however in the US that's completely legal. I'm not annoyed at the fact that I can't fly Nazi flags, and I would never do it, but a very similar thing does happen here, and that's our confederate flag.
The confederate flag is flown in many of the confederate states as a matter of pride, but to a lot of people it references a movement of racism and slavery. While I have no opinion on the matter, it's part of our free speech amendment to fly that flag if we so choose.
However even our might first amendment can be broken. This happened during the cold war with the communist witch hunt for anybody who agreed with communist ideas. You would be tried for treason, and locked up for a long time, and in some cases, executed.
Even still, American freedom of speech and expression goes beyond badmouthing the government. It's a rule that allows you to fly whatever flag, wear whatever costume, think whatever thoughts, and express those thoughts so long as it doesn't damage the lives of other people. This is something in many cases you can't do in most other countries, and while this hasn't come up that often, there is no irremovable rule in said countries saying that they can't arrest you for voicing your opinion.
Not to get to political here, even though this has already gone in that direction, my main gripe with this is the SJW insanity that has plauged the world. Not the people who are actually for free and equal treatment of everybody, but the people who harass other people, and give unequal treatment in the name of equality. There have been few cases yet, but a lot of people are falling for this cult of perpetual offense, and coming in to defend them. In the US I think my self largely safe as I have a for the most part respected constitutional right to voice my opinion regardless of the feelings of others, but there is no constitutional rule in, let's say Germany, that will prevent me from being held accountable for something of opinion or truth I said to someone.
This brings me to a similar matter. Jokes. I once asked my dad, who has spent most his life in Germany what he would do if a co-worker made a joke about Nazism towards him. My dad stated he would bring him to court, which would probably work in Germany, but not only would he lose the case here, he probably wouldn't even get a court to hear the case. His co-worker would probably get fired in both cases, but the difference is in the US he would not have to answer to criminal charges. This is another of my main reasons to have protected free speech. I want to be able to make a bad joke to anybody, and not have to face legal consequences. I would of course face social consequences, but this is between me and another person, not the government.
As a final note, don't take my continual example of Nazism to mean I support anti-semetic views. I do not, and am only using it as an example.