Those early general election polls are completely baseless! Candidates have not even debated each other yet. In addition, they are going with a base turnout model that is across the board inaccurate, so margins of error are far higher that they are reporting.
For instance, Establishment Hack Rubio wins primary and a fairly large portion of conservatives stay home, especially those that identify as Tea Party, as he has already burned them before. Cruz wins, they come out in droves. You cannot judge both of them as if they would have the same turnout!
Furthermore, Trump brings out an entirely different demo of Ind Voters that usually may not even vote in an election, not to mention Blue Dogs. He may even pull in good numbers with private union employees despite how the union leadership would be going with Hillary. This private union members want the economy to grow and Trump is the person they will most trust. Of course, public employee union workers will come up in large numbers as usual for Hillary.