Because of English's messy origins, I don't know if it's possible to say for sure
English is a language that traces word origins back to many other languages and, often, words from different languages were spelled similarly but sounded differently or sounded similarly but spelled differently.
There was a thing called the Great Vowel Shift that took place a little before the printing press was invented and which continued for a couple hundred years afterward. During this time, a lot of words were respelled (and their pronunciations drifted, too). Words like "room" were once spelled "roum," for example (because it originally came from the German word "raum").
So what may have happened was the word "to read" got boxed in because there were already words like "red" and "rid" and "rod" which limited its ability to have its spelling change when its tense changed. As a result, it ended up as a verb with an irregular conjugation.