It was originally designed as jpeg.
Some older operating systems (like DOS) can’t do a four-letter extension, they require a three-letter one.
So the three-letter one was used for those, and the four-letter everywhere else.
Nowadays you can use either one since most people’s systems are capable of using the four-letter one, but the desire to make things “backwards-compatible” is very ingrained in web design, so it’s still super common to see the three-letter one.
(Edit to add the word ‘some’ and similar verbiage changes as per corrections in replies.)
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