They’re words with etymological roots in Greek.¹ ἰδίωμα (and similar words that originated in Greek) is a neuter noun. When it was imported into Latin, it retained its grammatical gender (for Latin nerds, it’s third-declension so the genitive is idiomatis and not idiomæ as one might naïvely expect). When it entered modern Romance languages by way of Vulgar Latin, the neuter and masculine genders collapsed into a single masculine gender and thus it became masculine.

⸻

1. This is the case with many nouns that violate the expectation that an ending in -a would be feminine. The usual chapter one vocabulary word, nauta also comes from Greek, although in that case, the original word is ναύτης which is first-declension in Greek and remains first-declension in Latin, unlike idioma/ἰδίωμα.

¡Gracias!