Shared computing becomes a thing, and the need to have a better model of security is realized as a lesson from Viet Nam, and the Capability Based Security model is born.
Microprocessors again exponentially decrease the cost of computing, and Capability Based Security isn't required because all of the installations tend to have one or a handful of users.
The internet is born, and the cost of networking becomes exponentially cheaper, now all of those low security end users are connected together.
Systems become more powerful with the continuing drop in the cost of processor, memory and storage, so they become more complex. Nobody writes their own software any more, almost all coding is outsourced in some fashion. Security is only a concern if it trickles back to the original source as a problem.
A culture of "move fast and break things" pervades Silicon Valley, and the internet, and thus newer is always seen as better.
The lack of a security model at the base of all these systems is exploited for financial gain. Band-aid layers are added to try to patch the obviously inferior operating systems that pervade the land.
Because the lessons of capability based security were ignored for decades, and not taught, the common consensus is that computers can never be made secure, and your best hope is to hire the smartest people in the world, at less than the average market rate, to secure your systems.
And we repeatedly blame criminals, corporations, programmers, users, and now other countries, instead of solving the problem by properly implementing security.