It was less concentrated industry before the ACA.

the fundamental reason is as usual: too much government regulation that was erected with good intentions, but ended up becoming a competitive Moat that reduced the competition and entrenched big players. With that amount of regulation healthcare should be universal like in Canada or UK.

To maintain private healthcare sector, there should be less government

While I agree with this, that regulations are often championed and even written by entrenched companies to increase the barrier to entry, the government can sue to block any merger they deem a harm to competition. Unfortunately they choose not to.

AI is a good example of this. If the government regulated AI, the only companies who will control and profit from AI are the ones who already have it.