It should be noted that they increased the number, and had higher numbers in rural areas and for ethnic minorities. As for why, I'd have to assume it was related to why the restrictions on where someone could live and work stuck around and have just gradually been loosened: one of the single biggest material problems China faced after the revolution was a general lack of industrial capital, a lack of industrialized agriculture, and a constant struggle with rural poverty and the pressures that creates for people to leave and move to the cities; despite the rural conditions being bad, they needed a large chunk of the population to continue to endure them in order to continue to have food for everyone. That means they needed the population to have controlled growth while they industrialized and stabilized the food supply: with only so much farmland and so much industrial capital to work with, more people would just strain their limited resources even further.
Presumably it's only in the past decade that they've felt their economic growth is outstripping possible population growth and so they'll benefit or at least not suffer for increased population growth. This is also the timeframe for Xi's anti-corruption campaigns and anti-poverty programs, so it could also be an ideological change away from the cautious liberalism that had been the status quo towards what seems to be a more leftist focus on what the public wants.
So yeah, if anyone questions it just say the one child problem was a conservative liberal policy and repealing it was a popular communist policy and watch their brain melt in real time.
