This may not be the whole story, but one of the reasons is that JavaScript does not have a standard library. Corollaries are: several different module systems, application frameworks and bundlers exist.
Of course it depends on your system but it's more a matter of minutes than of hours. It's a really enjoyable experience to hack on the system and being able to rebuild in seconds. Just have a look at some SerenityOS hacking videos, they are truly inspiring.