Ten years ago most wallets were built around cash: a bill compartment, a coin pocket, several card slots as an afterthought. That design made sense when cash was the default payment method and cards were the backup.
As tap-to-pay and digital wallets took over daily spending, the cash compartment became the least-used part of most wallets, yet it was still adding the most bulk. The slim cardholder is really just a wallet with the now-unnecessary parts removed.
The people who still carry cash regularly haven't abandoned bifolds, and that's fine - the point was never that slim is objectively better, just that it finally matches how most people actually pay.
