When the last active Republican opponent to Donald Trump "suspended" his campaign, why did the remaining contests begin skewing heavily in favor of Trump? One could ask why voters who oppose Trump would bother to vote when the outcome is known, but one could equally ask why voters who support Trump when the outcome is known. Cruz (and quite a few other Republicans) are still on the ballot and, if people voted for Cruz, he would still win just the same as if he hadn't suspended his campaign. This happens every 4 years when a candidate in the party becomes the presumed nominee. Even though in theory other candidates could still win, the other candidates supporters stop voting for their guy and the presumed nominee starts getting nearly all the votes. This makes the remaining contests meaningless. Why does it happen?
When the last active Republican opponent to Donald Trump "suspended" his campaign, why did the remaining contests begin skewing heavily in favor of Trump?
The contests started skewing towards Trump (New York through Indiana) and then Ted Cruz and John Kasich suspended their campaigns. They may be skewing more heavily towards Trump since the suspensions, but the skew began first.
Prior to New York, Trump hadn't won a majority of the vote in any state (he did win a majority of one of the territories). Starting with New York, he has won almost all the delegates in each state. Mathematically it might be possible for Trump to fall short of 1237 delegates (one more than half of 2472--the total number of delegates). Realistically though, he would have to lose some of the states that he is expected to win. And he would have to do exceptionally badly with unbound delegates.
Note that Trump had 1014 delegates when the other two dropped out. But this undercounted his real strength. For example, in Pennsylvania, 54 of the 71 delegates are unbound. They could vote for anyone at the convention. But most of them have promised to vote for the winner of their congressional district. So Trump can expect to get at least 40 more. He is also expected to win New Jersey, which is winner-take-all (51). That's 1105 delegates. Plus there are 394 delegates to be awarded in other elections. Trump barely needed to win a third (132) of those to make it to 1237. And there's another fifty or so unbound delegates who could be wooed if he fell just a little short.
Since Indiana, there has been little to no hope that Trump would fall short of a majority of the delegates.
Without the candidates in the race, there is little incentive for their voters to go to the polls. The candidates have essentially told them not to bother. And they are no longer pushing supporters to the polls with direct mail, phone calls, etc. The end of the campaigns means that there is no more money for this.
Finally, there are some people who just like supporting the winner. The contest is effectively over, so just go ahead and make it actually over.