IMO, bad actors.

40 years ago we didn't need much in the name of cyber security, restrictions, controls, etc. You most likely personally knew the vast majority of other people that had any kind of access to your systems, or that you exchanged any kind of electronic data with.

As the aperture of users and developers has opened, risks have increased. There is probably some analogy here similar to how the value of network increases exponentially with the number of connected devices, the risk factor increases exponentially with the number of users and potential software developers for an ecosystem.