All the primaries for the Democrats use proportional representation. That means if you get 30% of the vote then you get 30% of the delegates in that state. Not so with the Republicans. Many of the Republican primaries are either winner take all or winner take most. That means if you get 30% of the votes and win because no one else gets as many then you get 100% of the delegates in that state. What is the effect of this? Well, it means Trump could lock up the nomination without ever winning 50% of the vote in any state. Romney essentially did that. Trump knows how these numbers work. It is to his advantage to have multiple adversaries. All he has to do is have a few more votes than the closest competition.
The real problem that the republican voters are having is that they have no viable candidates to vote for. And that's the plain truth.
Even as absurdly lockstep as republican voters tend to be, and as willing to vote for satan himself rather than any democrat or independent as they are, the party honchos still can't find a candidate that's realistic to even
that willing of a constituency. Let's face it. No one actually believes Donald Trump is a qualified and reasonable choice for president. And yet he continues to lead in the republican polls because the others are equally unqualified, and equally ignorant and dishonest. So the deeply partisan republican voters are sticking with Trump simply because he's the best known of the idiotic collection of horrible candidates being offered to them by their party. And they think that perhaps his notoriety could get him elected. Whereas the others really don't have a prayer.
Frankly, I think republican voters are getting what they've deserved for along time. They've allowed themselves to be grossly manipulated for decades by their own ignorance, prejudice, and bile, and now their getting tossed a collection of ridiculous candidates, as a result, because the republican party honchos know they'll vote for whatever crap they throw at them.
Unfortunately for the republican honchos, Donald Trump knows it, too.