You cherrypicked one poll.
Now take a look at all of them.
https://en.wikipedia.org/wiki/Nationwide_opinion_polling_for_the_United_States_presidential_election,_2016Go back to March and April when multiple candidates were being included in the polling, and compare.
Apples to apples—with conditions being the same for all the candidates in regard to the initial conditions—Trump consistently performed the worst of the three remaining candidates at the time. Against Hillary Clinton, here's what each candidate got in each poll:
NBC/WSJ, April 10-14: Trump 39, Cruz 44, Kasich 51
Fox News, April 10-13: Trump 41, Cruz 44, Kasich 49
CBS News, April 8-12: Trump 40, Cruz 42, Kasich 47
Investors Business Daily, March 28-April 12: Trump 35, Cruz 39, Kasich 45
Marist, March 29-31: Trump 41, Cruz 47, Kasich 51
Public Policy Polling, March 24-26: Trump 41, Cruz 42, Kasich 45
Bloomberg, March 19-22: Trump 36, Cruz 42, Kasich 47
Quinnipiac, March 16-21: Trump 40, Cruz 42, Kasich 47
New York Times, March 17-20: Trump 40, Cruz 44, Kasich 47
CNN, March 17-20: Trump 41, Cruz 48, Kasich 51
Monmouth, March 17-20: Trump 38, Cruz 40, Kasich 45
Even controlling for any persistent bias, compared to Cruz and Kasich, Trump consistently performs the worst of the three candidates in all of the general election polls, not just one—something that has remained stubborn over time.
This, Mechanicos, is what they call the "scientific method."