Why is this thread locked?

I would assume because of the reactivity of vaccine discussions. IMO, most people would not be able to process the distinction form a "problem with vaccines" versus "a problem when vaccines are manufactured poorly". Despite the weak connection to UC, the potential for a divisive non-productive thread was large and any potential benefit to a UC forum seemed small. But this is IMO - I am not a mod.

By analogy, if one found a rare contaminate no longer used in toothpaste formulations had in the past caused some gum disease, and then argued that toothpaste should be banned because toothpaste is bad and we cannot trust the toothpaste industry at all.

Obviously toothpaste is not bad, and even if there have been transgressions and bad influence of corporate greed, we can trust toothpaste manufacturers overall.

The reason was given as the last post on that thread, I suggest you read that Again, this is not a vaccine forum, there was a lot of heated back-and-forth, and nothing good would come out of it if let go on (as heels dig in and people get defensive about their point of view, the insults get worse and worse, and inevitably turn into a flare war). It was allowed to go on for a while as a compromise, but is better discussed in vaccine-themed forums of which there are options for those who are interested within that subject. Cheers!