They never were. Conservatives used to hate populism.
Europe has different populist parties - some had Communist roots like Greece’s Syriza, some came from fascism such as France’s National Front and some are conservative, like the UK’s UKIP.
What they all have common is a rejection of globalism, neoliberalism and multi-culturalism. So despite differing ideological origins, populists all arrive at basically the same place.
Donald Trump is not a conservative, he is a populist on the right side of the political spectrum.