Digital money had a copy-paste problem
A physical dollar bill can only be in one place at a time. Hand it to someone and you no longer have it. Digital files don't work that way by default, a JPEG or a text document can be copied infinitely with no loss of quality. Before Bitcoin, this was the core obstacle to digital cash, if money is just data, what stops someone from copying their balance and spending the same units twice, once at a store and once somewhere else? This is called the double-spending problem, and for decades it had exactly one known solution.
That solution was a trusted third party, typically a bank or payment processor, keeping a private ledger and checking every transaction against it before approving it. It works, but it means every payment depends on an institution you have to trust to be honest, stay solvent, and stay online. Bitcoin's insight was that a public, shared ledger, verified by many independent computers instead of one trusted company, could solve double-spending without needing anyone to trust a middleman at all.