“Why are Jeb Bush, Carly Fiorina, Rand Paul, John Kasich, Chris Christie, Mike Huckabee and Lindsey Graham still in the race?”
Whatever the reasons that they are still in the race, their presence is performing the commendable function of splitting the RINO vote into multiple, tiny fractions. I hope they all stay in the race until the bitter end, as it is a tremendous help for Trump.
It's interesting to consider what the polls will look like if everyone other than Trump, Rubio, and Cruz were to drop out.
Nearly 100% of Jeb, Carly, Kasich, Christie and Graham supporters would go to Rubio, giving Rubio an additional 17 points if you trust this poll. I'm not sure where Paul's 4% would go, maybe half would go to Cruz, the rest who knows. I would guess that about half of Ben Carson's supporters will go for GOPe, the other half will be evenly split between Trump (those who want a non-politician) and Cruz (Evangelicals). A good portion of Huckabee and Santorum's vote will go to Cruz (even though Huckabee and Santorum are liberal on non-social issues), the rest will go to Rubio.
So my guess as to what the polls would look like if Carson, Bush and GOPe also-rans dropped out is (very roughly, due to not really knowing where Carson, Paul, and Huckabee voters will go) this:
Trump: 40%, Rubio: 35%, Cruz: 25%