Huh, isn't that kind of obvious?

A big part of left wing ideology is progressivism, which is an exaltation of progress. The exact definition of progress changes over time, but the idea is at heart about having the most intelligent and specialized people being granted power to re-organize society for the greater good, as they see it. It is explicitly against the public making their own decisions in a decentralized way (neither via markets nor politicians), because it's assumed they're too stupid to do so with good outcomes.

Core to this worldview is the belief that there are such people as neutral experts, wise intellectuals, morally uncorrupted regulators and so on. You have to believe these people exist and in large numbers, otherwise the core progressivist project falls apart because who would be in charge of society? So the left are very keen on regulators, NGOs and especially "scientists", by which they don't mean corporate scientists but academics and government lab workers. They see in such people the core of the progressive project. Motivated by the greater good rather than self-centered profit, they deploy their superior skill and wisdom to make the world better.

So COVID comes along and it appears at first to be the perfect vehicle for validating the progressive project. Nature attacks, and who will defeat it? Not politicians or ordinary people! It will be the scientists and public health officials - but they say they can only do it if they're awarded total power over our lives, if politicians and markets are disposed of entirely and if we're all forced to act in concert (so we get collectivism as well as progressivism). Ideal!

The lab leak conspiracy turns this progressive dream into a nightmare. Suddenly the experts, the public health officials etc aren't the heroes of the story anymore, they're the villains. It turns a story about the virtues of progressive power into a story about how supposedly intelligent people killed millions through a foolish, careless and ultimately useless pursuit of abstract "progress" without regard to risks. And if they lied and covered up that then what else might they be lying about? How can anyone defend these people as superior leaders when they behave this way?

So depending on whether or not it came from a lab, the conclusions can switch from total validation of left wing goals to total repudiation. Of course they will never accept that it came from a lab leak.