Here’s an old news article that talks about why English has so many irregularities when it comes to the past tenses of common verbs (e.g. “I am” -> “I was”, “I see” -> “I saw”, etc.). As it turns out, the reasons aren’t too surprising – like so many things in English, it relies on common usage.
The article compares such linguistic evolution to biological evolution – after all, the most often-used (and therefore useful) genes generally stay the same while the rest of the organism evolves. “To help” isn’t as common a verb as “to be” or “to have”, and so is more open to linguistic evolution – that is to say, the more frequently a word is used, the more it will change over time. The original past tense of “help” was an irregular form – “holp”. However, since the verb wasn’t used as often as other more common verbs, people tended to forget about the ‘special’ nature of the past participle, and simply lapsed into the ‘regular’ way of rendering a verb in the past tense – adding -ed to the end. Thus, over time, “holp” became “helped”.
Lieberman was struck by this idea when he learned that the ten most common verbs in English (be, have, do, go, say, can, will, see, take, get) are all irregular. Instead of their past tenses ending in ‘-ed’, as do 97% of English verbs, they take the peculiar forms of was, had, did, went, said, could, would, saw, took and got.
Researchers suppose that this is because often-used irregulars are easy to remember and get right. Seldom-used irregulars, on the other hand, are more likely to be forgotten, so speakers often mistakenly apply the ‘-ed’ rule. The most commonly used word that they found this happened to was the verb ‘to help’ – the past tense used to be ‘holp’, but is now ‘helped’.
You can read the whole article here. Very interesting stuff!