'Why?'
Because Trump is not going to let some Establishment type oppose the policies that he wants to carry out to "make America great again".
'I thought they were already "for the people"'
Yes, the head of Dow Chemicals certainly is, but some of teh servants in the political class were servants of the elites and not the people, but now that is all over.
'So, are they "for the people" or will they be forced to be by Trump?'
Trump is for the people and they are serving under Trump, so they will have to be for the people because Trump has defeated and humiliated the elite. That is why Trump met Farage at Trump Tower and Cameron hasn't been asked to meet Trump.
'And why not choose people from worker's unions, for example? '
Because as Trump said, the union offical at Carrier did nothing to save the people's jobs at Carrier. Trump wants top business people who know how to make billions of dollars and know how to create business and jobs, not some union officials who sat back and voted for the Clintons while they served the elites by offshoring the people's jobs.
'People who have been working for NGOs?' 
Soros NGOs? It's all over. You'll say what about people working for charidees next? This is not New Labour, this is Trump. The game's up.