Because the rest of the Republican Party was filled with limp-wristed, squishy, go-along-to-get-along types.

As I tell people: I voted for Trump to destroy the existing GOP. To date, he's done a pretty good job of it.

Before Trump, people in the GOP tried to be nice. The "Tea Party" election of 2010 (and to a lesser extent, 2012) was sidelined by the GOP leadership as being "too conservative" and "too inflexible." So the GOP leadership insulted, demeaned, and hazed out the "Tea Party" types, and continued on with "business as normal" in DC.

Trump comes along, and from the first debate, we saw that Trump wasn't playing by the rules. He was going to go forth into debates and be a pugilist - someone who was not going to "go along to get along."  Trump proceeds to take down Jeb! Bush, who had a $110+ million dollar war chest going into the first primary debate. Trump quickly dispatches the Bush dynasty ideas, and then the GOP base noticed he was an actual threat to the entrenched idiots of the GOP.

Trump then goes on to dispatch the rest of the field, as a pugilist, and then, against what seemed like insuperable odds, takes down Hillary Clinton at half the money spent on the election. Remember, going into Election Day 2016, all the "experts" were saying that Clinton had a "92% chance of winning."

Now the GOP realizes that Trump is basically unstoppable by conventional means - the old equation of "the candidate with the most money wins elections" no longer holds. Now the GOP and DNC knives come out for Trump, because he's upsetting the applecart.

At this point, there was no one like Trump in the field. No one. Quickly the GOP diverged into two camps: 

- The "establishment" camp, who backed people like the Bush clan, and who were wise to the grift and insider-dealing in DC, and didn't want anyone upsetting the applecart...

- And Trump, who by being unconventional, gave people some hope that he could actually change things.

Now, at the pace of the last two months, there is no one else. No one else has put on that much change, has done so much to expose the rot in DC, as fast or as deeply. Now the GOP insiders know that they're done, and a new generation of GOP leadership has to sort itself out.

What is really obvious to the GOP base is that the era of faux intellectualism (hewing to William F. Buckley, and his ilk quoting intellectuals) is over. The GOP is now the male-dominant, and working-man dominant party. There is a huge shift in the base of the GOP that the GOP old guard does not know how to handle.