Because it's utter bullshit. All of the polling tells us the Trump voters skew higher in wealth and wages than Clinton voters and that hate for "the other" is largely motivating their choice. It's a narrative Republicans love because it gives them some tie to authentic "real Americans" and an argument to deflect from their pure, naked racism and neoliberal Democrats love because it allows them to pin Trump on the working class, but it's completely wrong.

You accurately describe what I've seen of primary polling, but not what little general election polling I've seen that breaks out income levels, e.g., http://www.people-press.org/2016/08/18/1-voters-general-elec...

Read the poll, this still shows that Clinton has a larger slice of low income voters. Also who do you think Trump's base is? The people who voted for him given a pick at any of the potential nominees for president, or the people who had additional choices eliminated and were encouraged to vote based on partisan loyalty?

Also I've included polling data that is past the primary, and includes voter sentiment for the general election, such as the gallop poll. The trend towards upper-middle craftsmen jobs remains.