My understanding is that for most package managers the signing keys are held by a smallish number of maintainers responsible for entire sections, who presumably keep those accounts pretty tightly secured. Not impossible to take over, but it's a smaller attack surface.

While for NPM as far as I know every uploader keeps their own account and there's not even signing keys to lose control of.

Trust by default for a atomic packaging system. Entirely NPM's fault.

It's a "package manager" that has zero integrity checks built in. Web devs also love it. Nice combination.

because it's the biggest. Just like how hackers target windows and not linux (assuming they are targeting users and not servers).