In my (non-expert) opinion, there are a few reasons

1. NPM is more popular than those other services by an order of magnitude, especially among new developer and startups.
2. NPM allows for code to be executed while you install the package which is different from maven or nuget and allows for easy exploitation paths

Mods of communities can already see votes in communities they moderate. Admins of instances can already see votes on all content.

These ads only appear in the “promotions” section of Gmail, the section that is by definition for advertising emails. It’s not great, but this is the least intrusive place to put ads.