The reason for that is structural. The Democratic and Republican parties have structurally built themselves into the election system in a way that shuts out any attempt to gain traction by third and fourth parties.
Just think about the fact that the Federal Election Commission, the organization that oversees our elections, is legally required to be half Democrat and half Republican. That tells you everything you need to know.
If we're going to change it to make room for additional parties, we have to take over one of the existing parties with an explicit election reform agenda that removes them from the structural landscape and then restructures it (with things like ranked choice voting or a senate that works on proportional party representation) to allow for third parties to gain traction.