Also, polls are just snapshots in time. This process of attitude formation and change keeps me in business. You often see race and sex. Most polls do ask questions about income and education level, and many stories do break down results by income, education, marital status and other important variables. That doesn't give much time to spend much time with looking inside the survey results.
Space is another problem.
We often just don't have enough room to include much beyond the overall results in our stories. We media pollsters remain data rich, and analysis poor. There's more evidence for your hypothesis is elsewhere in the new Post-ABC News poll. Among those who don't want Clinton impeached, half say it's because they don't think he did anything serious enough to warrant impeachment. But the other half say they don't want to put the country through the trauma of the impeachment process, irregardless of what he's done.