Back then there was no versioning. When you imported a URL, you had to hope that all your projects and transient dependencies happened to be compatible with whatever commit was currently sitting on the master branch. Because one update could silently break an unrelated project on your machine, adding a dependency was a bigger decision. That naturally led the ecosystem to favor fewer, larger libraries.

PS: it looks like this issue was addressed in 2019 with the introduction of Go Modules.