I don't know the percentage of registered Democrats, Republicans, and unaffiliated voters, so I can't give precise figures, but this is my general impression:
Neither Democrats nor Republicans have a majority of registered voters. To win, both sides need to attract unaffiliated voters.
Both parties have almost enough partisan voters to win, but they need unaffiliated voters (or crossover voters) to put a Democrat or Republican candidate over the top.
It's like three glasses of water. Both Democrat and Republican candidates begin with a glass that's fairly full. So they just need to make up the difference by siphoning water from the third class to top off their glass.
Let's say Hillary becomes the Democrat nominee. She will basically get 100% of Democrats.
Let's say Cruz or Rubio becomes the Republican nominee. He will probably get about 100% of the Republican vote.
Suppose Trump is the nominee. He will get far less than 100% of the Republican vote. From what I've read, many conservatives have said they won't vote for Trump no matter what.
So the initial water level for his glass is far below Hillary's. He starts much lower. That means he must get enough unaffiliated voters just to get back up to the level where Cruz or Rubio begin, plus enough additional unaffiliated voters to put him over the top.
By contrast, Hillary has to get far fewer unaffiliated voters to put her over the top, because her glass is so much fuller to begin with. And they'd be competing for the same unaffiliated voters. It seems like Hillary has a strong completive advantage under that scenario.
Suppose Cruz or Rubio becomes the nominee, and Trump runs as a third-party candidate. There will be a two-way split in the Republican vote between Trump and the Republican nominee, as well as a three-way split in the unaffiliated vote between Hillary, Trump, and the Republican nominee. It seems like Hillary would be undefeatable under that scenario.