Why was Millennial liberalism so short-lived?

That’s simply not entirely true. 2016-present was not dominated culturally by conservatives all around like in the  different decades, it’s only a segment of the population with a certain limited degree of influence, of very different and more liberal/flexible kind of conservatism, which has been decreasing, if they had any strong influence enough to change culture,  social attitudes among the average person today would be closer to 1955 than the present ones you see. 

The 70s weren’t as consistently liberal. Not all boomers are conservative, you can’t really paint them all with the same brushstroke, they are the ones that established modern liberalism. The 90s were in between liberal/moderate , very liberal in certain regards but with some degree of conservatism still lingering.

You can even argue that 2004-2010, despite leaning a bit more conservative in many ways, were transitional and inbetween old school conservatism and the liberalism we have today. This can be traced to the early 2000s.