Here is what Senator John Kyl from Arizona says happened in a private conversation that Obama told him why Democrats refused to secure the border.
The White House denies the account. Senator Kyle refused to back down from his story, saying “They want to get something in return for doing their duty. You don’t have to have comprehensive reform to secure the border, but you have to secure the border to get comprehensive reform.”
Meanwhile, here's an example of the federal government's "work" on border security: