While regulation will always play a role in this, I would hesitate to say that this is "the" difficulty. Transferring dollars (say) electronically is complicated because you need to prove that no dollars are being created or destroyed. Though you could phrase this as a regulatory rule, it's really more of an accounting requirement.

The system as it stands (FedWire/CHIPS/SWIFT, and ACH on a consumer level) is very complex and tries to strike a balance between reasonable clearing times and limiting trust of individual actors.

The difficulties in an electronic money transfer system for fiat currency are, in my option, primarily technical and logistic, with the logistic aspect being maintaining the correspondence between specie and electronic balances.

That said, it's always hard to analyze success. Arguably, bitcoin does not solve the money transfer problem any better than previous schemes, like Chaum blind signatures or egold. The contribution of bitcoin to the electronic payment space is a little more nuanced than just "easy money transfers".