It is widely accepted that the English word "data" in its computer sense is most commonly used as a mass noun (like "bread", "sand" or "money").

Part of the reason for this is that it is impossible to quantify a "datum" in computing terminology. Is it a bit, a nybble, a byte, a word, a float,... an ASCII character, a Unicode character, a character string.... It is each of these things and more, depending on the context.

Given the variability of the term, it is essentially meaningless. How can a word exist without meaning? So the word "datum" died, leaving "data" as an orphan. How can you have a plural without a corresponding singular? You can't -- and thus "data" became uncountable.

There are academic (non-computing) contexts where a "datum" still exists, and in such contexts "data" is treated as a plural, but these are rare and in a very restricted domain.