It's probably a regional thing that you're more used to diminuendo. The also-subjective version, that I got taught here in Germany, is that the hairpin notation (< and >) correspond to crescendo and decrescendo, and that diminuendo is just an alternative way of saying decrescendo.
I'm guessing, they taught it to us that way, because just adding "de-" to negate is easy to remember. Maybe native Italians do use "diminuendo" more naturally. It certainly seems less unwieldy, because it doesn't use the negation.
But yeah, ultimately it's like how English has "silent" and "quiet", which mean essentially the same thing, but both are in use. If we hadn't already standardized on "piano", you'd probably find both words in English compositions, with whichever one used that the composer liked more in that position.