The "i"s is these names are pronounced differently. How did this come about? Maria: long e sound (bee) Mariah: long i sound (nine)
They have different histories. Maria came to us from the Romans, who got it from the Greeks, who adapted it phonetically from Hebrew Miryam, the sister of Moses. The Hebrew word is ambiguous and could mean many things, from "beloved" to "bitterness." The original form, now spelled Miriam, is also used by anglophones. The most common form of Maria in English is Mary. The French form is Marie, with the standard Gallic accent on the final syllable. It's also quite common in English because of the French superstratum in our language dating to the Norman invasion. Mariah is of Middle Eastern origin and first showed up in England about 500 years ago. It could be the same name as Mary/Maria/Marie/Miriam, having passed through other Middle Eastern languages and enduring phonetic alterations of its own. Or it could be derived from the Hebrew place name Moriah, since vowels are quite ephemeral in the Semitic languages. In any case, both names come by their vowels honestly. Neither is a made-up name like Dweezil or a made-up spelling like M'shell.