I don’t know why Americans are surprised by this kind of thing. It’s a natural result of having the primary system that they have. In the American model, primaries are a way for the most extreme wings of any caucus, if they hold a sway vote, perhaps as small as 5% or even less, to push a more radical candidate.
When you have a party like the Republicans, whose policies historically have been unsupported by a majority of US voters, often even within the Republican Party itself, you create the situation where the prevailing candidate for election will have to make commitments to single-issue factions to gain support, and that’s been exacerbated by the rise of Trumpism.
You can see a similar mechanism at work in Israeli elections, where the proliferation of parties happens because their promotional representation electoral system allows parties with as small as 5% of the vote to take seats in the Knesset. To rule, parties with more moderate views have to compromise with more extremist factions.
In many other countries with primaries, the organization of the primary is within the party itself, and most parties take steps to limit participation to those who have established standing (i.e. a minimum of one-year registered membership in the party) to allow participation. Because the US system is a matter of legislation, anyone can “join” a party simply by changing their affiliation – another absurd way of doing things.
And in better-run democracies, run-off elections ensure that final electoral contests produce an actual popular winner, with the majority of the voters behind them, erasing much extremist influence. and others simply do without it, and things work fine.
Primaries are the cheapest place for corporations and PACs to exert influence, by effectively reducing the number of candidates in the actual inter party elections they have to buy. And primaries, coming before elections, act to guarantee that voters are exhausted by campaigning that never ends months before the actual election; while doing nothing to actually focus on general popular demand; parties run their primaries solely on the issues and policies that they want to focus on; where was the last Republican Primary that was won or lost on Climate Change policy?