It's because of the effects of technological progress on wealth accumulation.
Tech allows winner-takes-all effects in many different markets (and runaway situations where labor cannot catch up at all) - you either have to have very strong antitrust (which is still not possible in some places), or basically strong redistribution (i.e. taxation of wealth in order to redistribute) to maintain a reasonable (note: not equal, but a reasonable degree of inequality - closer to the model that existed from the 1940s-1970s) distribution of wealth in society.
Tech allows winner-takes-all effects in many different markets (and runaway situations where labor cannot catch up at all) - you either have to have very strong antitrust (which is still not possible in some places), or basically strong redistribution (i.e. taxation of wealth in order to redistribute) to maintain a reasonable (note: not equal, but a reasonable degree of inequality - closer to the model that existed from the 1940s-1970s) distribution of wealth in society.