I agree with everything above, but I would like to sum it up in my own words: we (as a species) were so new at writing software, that we had no idea of its inherent complexities. Programming was an after-thought for those that designed the first computers.
As soon as we started programming, we found to our surprise that it wasn't as easy to get programs right as we had thought. Debugging had to be discovered. I can remember the exact instant when I realized that a large part of my life from then on was going to be spent in finding mistakes in my own programs.
— Maurice Wilkes
I think that one of the differences is that, in 1954, computer programs were single entities written by one person or by a team working closely together. So the "between two pieces of working code" bugs-- which tend to be the nastiest kind in modern development-- weren't really seen yet. Million-line codebases weren't even on the table as a reasonable concept, and the idea of a program depending on 120 other libraries or frameworks was unimaginable.