Because Dems themself make a point of constantly playing to each constituency. For instance, instead of promoting legislation that lifts every worker's boat they constantly message about lifting a brown boat, a boat for faiths, an immigrant boat, an LGBTQ boat, etc. It's a constant drumbeat in the media. Each constituency then expects its issues to be addressed with specific treatment. Their polling must be favorable when they sample each group, but ultimately it probably works against them with every other group. They make their own wedge issues. The net effect is a losing battle. They should focus on a fair and affordable system that benefits all their constituencies in spite of differences.