IMO the shift to the right has more to do with the electorate itself than the system, nobody is forcing people to vote for Trump.
I've seen some studies showing that just the simple act of having more minorities around automatically makes white people vote more conservatively.
So as long as demographics keep changing that section the country is going to keep moving right.
I don't know all the technicalities but it was my understanding that the whole gerrymandering thing benefitted Republicans, for example. But on a much larger scale, the very fact that you vote for someone as opposed to for a party or a program leads to the culture of the leader, which benefits more the right with its hierarchical way of seeing the world, mose so than the left where things are (were) more horizontal. Add to that that in order to be a viable candidate you need tons of money, and that also benefits the right and its natural alliance with big business. The fact that minorities will push white voters to the right isn't something "natural", but because the poiltical discourse, the way countries/nations are set up since the 18th century and the media (controlled once again by big money) have created the "us" vs "them" rhetoric. (Even more so in the US, due to slavery). In countries such as France or the US, in theory citizenship trumps the rest. We all know that isn't the case, and that's because behind that discourse there is a political construction of "us" vs "them". A counter-example would be some multi-national empires that existed before the nation-state, in which "the Other" was not necessarily the neighbour that looks different than you do (I'm not saying that everything was perfect, far from that, but the lines could be different). You can even go further as to how trade unions have been all but annihilated in most western countries, and that "free" market policies are hardly ever questionned even by people seen as being on "the left". That's not a "natural" evolution, which is what the Reagans and the Thatchers of the world wanted/want us to believe ("There is no alternative") but a political ideology.
You're right, no one is forcing people to vote for Trump. But decades of media pressure, the way the system is set up (Trump and in general the right present "simple solutions" to complex issues that people sometimes don't have the education or the time to fully understand, because education and time are costly...by choice) and the fact that anything even ressembling "socialism" is constantly vilified, it's much more likely that they will. And the other party knows that, so they end up sending a candidate that is not radically different, but is kind of more acceptable and will not fundamentally change much.