Italian-American philosophy professor Massimo Pigliucci explains this, and writes:
In the early part of the 20th century philosophers of science were looking for ways to explain why science is an objective enterprise. Think the logical positivists, or Karl Popper. Then came the so-called “historicist” turn, with Thomas Kuhn and Paul Feyerabend, and philosophers finally realized that science is not, in fact, intrinsically objective at all. (Many scientists haven’t matured to that point yet.) ...Once you start going down the path of denying the existence of objective truth, then you usually end up babbling nonsense.
Of course, all of the above matters much less now that the United States has elected a fascist to the Presidency and given absolute control of power to a bunch of regressive sexists and homophobes.
It is very strange to claim that historicism shows that science is not objective. The history of science is primarily a story of man finding objective truths.
Some ppl argue that in physics, relativity and quantum mechanics killed the idea of objective truth. No, they did not. They reinforced the virtues of sticking to objective truths.