Back when it was a Latin word spoken in ancient times, there weren't many data. This is my theory. In those days data were in fact countable. So you could count data, and when you had one, it was a datum.
However, times have changed. And we can clearly see from the graphic I want to share with you, that data has, thanks to our technology become as uncountable as sand on the beach, the water in the sea and time itself. Data now definitely is, as opposed to are and there is no longer a datum but rather a piece of data.
I read this article from Vice, it is pretty detailed and gives quite sensible explanations about the shift in usage and meaning. We have "semantic bleaching" where the meaning of words can alter slightly over the centuries and this seems to be the main reason.
It's Time to End the 'Data Is' vs 'Data Are' Debate