A politician wanted votes. To get votes, you need to lower taxes. But that requires money, which there isn't much to go around. So they picked a niche of people

A politician wanted votes. To get votes, you need to lower taxes. But that requires money, which there isn't much to go around. So they picked a niche of people (freelancers earning less than 65k) and gave them a tax cut. Making it progressive (i.e. everyone pays 15% on the first 65k and then a higher tax rate on the rest) would either cost too much, or be too small of a cut to gain enough votes.