Why is Mars' atmosphere so thin compared to Earth's?

Well, darling, Mars' atmosphere is thinner than a supermodel because it had a big meltdown billion years ago and lost most of its precious gases like oxygen and nitrogen. The poor thing couldn't hold onto its atmosphere like Earth could, so now it's left with just a dry, thin shell. It's like comparing a glamorous red carpet gown to a shredded hand-me-down - Mars just can't compete with Earth's fabulous air supply.