The party decided
The Party Decides is a 2008 political science book, which argues that presidential nominations in the US are controlled by party establishments. This used to be literally true, but the book's claim is that since popular primaries were introduced in the 1960s and 70s, the parties have developed ways to support their chosen candidate, who usually ends up winning even when they weren't initially favoured by voters.
It's come in for a bit of a pasting in the last year and a half, for pretty obvious reasons: the Republican Party establishment did not want Donald Trump, and they signalled that very clearly, and yet. You can read various bits of musing about that here, here and here.
What if the party did get what it wanted?
There was a time when Trump looked like he might run a campaign that was unusual ideologically as well as rhetorically. He talked about raising taxes on the rich, increasing Social Security benefits and expanding government-provided healthcare; on more 'cultural' issues he was not only much more crude but also much more substantively anti-immigration, importantly including legal immigration, than the Republican establishment. He was an anti-intervention, 'America First' isolationist on foreign policy. This is not a great platform: it's essentially the kind of traditionalist welfarism that fuels the Front National in France or UKIP in Britain. (Some of the time, anyway - UKIP is famously changeable on most issues.) But it's also clearly not the ideology of the mainstream Republican Party.
These unorthodoxies were mostly gone by the end of the primary campaign, forgotten about by the end of the general election, and are completely non-existent by now. Attorney General Jeff Sessions is down on the border launching an immigration crackdown, focused wholly on irregular immigration and even then on enforcement measures that don't directly disrupt business operations. Trump didn't seem particularly interested in the healthcare bill that died in Congress a couple of weeks ago, but he's not interested in much; he was publicly and privately supporting a bill which would massively cut taxes for the wealthy and strip 24 million people of health insurance. He's declared a policy of regime change in Syria and is happily presiding over the same kind of militarist drumbeat that led up to the invasion of Iraq.
None of this is an accident. Even if party establishments can't control who gets votes in primaries, partisanship is the engine of American politics. People change their issue stances based on their party identification. Policy advisors, political staffers and even journalists progress by becoming affiliated with and known within party circles. You can, if you have a lot of money and a bottomless desire to make outrageous subtext text, launch a hostile takeover via a party primary. But in the end, you still need the party. You need its regular voters - the ones who don't care enough to vote in primaries and have no great enthusiasm for you in particular - to get you over the line, so you can't afford to diverge very much from the standard ideological formula. You need its congressional representatives not to turn their institutional firepower on you, so you can't alienate them or reject their priorities. You need people to do your comms and write your bills, and the party is the only place you'll get them, so - whatever you want - your bills won't end up that different from what any other president from your party would have proposed.
The Republican Party didn't get to decide who'd be its nominee, but by a variety of formal and informal mechanisms it was able to enforce Republican ideology on whoever that nominee was. If not in quite the way The Party Decides envisions, the establishment is in the driver's seat, and we have a White House which - though probably more openly bigoted, and certainly more of a circus - is doing roughly what you'd expect a Republican White House to do.