One other reason translations took off is that last year the translations for plugins and themes moved to GlotPress. Before these were maintained by the plugin and theme maintainers and included in the ZIP-files.
I would assume more people found their way to GlotPress and felt eager to contribute to WordPress Core as well.

As a plugin maintainer I am quite happy that this happened, it eased the burden of maintainers a lot.
I also hopes this improves the quality of the translations, as more people have good access to the translated strings.

One concern I have is that many languages have quite a backlog of entered strings that are waiting for validation.
It would be good if more people applied to become translator and validator (PTE) for plugins and themes they care about.