I had never heard anyone use "oftentimes" as a word until I watched an American in a youtube video about 5 years ago. I am confident that where I am in Australia and in the UK that it wouldn't be standard English; if you wrote it in a school essay you would gte a red mark. So from the wiktionary:
inherited from the Middle English oftentymes; equivalent to oftentime + -s. Compare oftentime.
So if it had an origin of Middle English as above, rather than an American origin, then why did oftentimes not become standard English in the UK, Australia or other parts of the Anglosphere?