The average democrat doesn’t have a job, doesn’t want a job, and doesn’t pay federal taxes. For them the tax cuts didn’t work because they measure success by an increase in their welfare checks, not a job or a decrease in taxes they never paid in the first place.
For the target audience of the MSM it’s easy to convince them the tax cuts didn’t help them because it was directed at creating jobs and letting workers keep more of their own money, neither of which are a desire of the welfare class.
Most democrats won’t vote for Trump in the first place, whether the tax cuts help them or not.