Font Plugins and Google Fonts Popularity

I am eagerly curious to understand why Google Fonts is such a popular source for font plugins when there are other sites that also offer Open Source fonts, but many more of them than Google Fonts?

I’m not complaining, I just fail to see why someone who is going to the trouble to design and build a plugin wouldn’t choose a source that offered their prospective users the greatest number of fonts possible to select from? There must be a reason, some logic to this observed phenomenon and I’d appreciate it if it could be explained to me.