One potentially contributing factor is that npm makes it very easy to avoid conflicts with duplicate dependencies, i.e. if one dependency has a transitive version on some other package, and another dependency also has a transitive dependency on that package, but on an incompatible version, that's not a problem in npm: it'll just store both on your disk. That might remove some pressure to minimise the number of dependencies on library authors.

And of course, there are just far more people working on it, I believe.