It's a combination of a lot of things...
There was no script department on the show as such, so aside from old breakdowns and synopses, there was very little written material to refer to. With no process of keeping records, it meant that things that were changed after the writing - either by accident or design - would often go unrealised by the writers. There were no tapes to refer to, and locating specific stuff for cross-reference was incredibly difficult. So it all hinged on people's day-to-day memories, which were patchy and incomplete - certain writers would drop out for months at a time, be replaced, etc, too...
Similarly, with sometimes just hours to learn a script, the actors rarely had time to worry about the minutae, and would assume that the stuff was correct. This is still the case on soaps today - it's another issue of time. Assuming someone did spot a gaffe, it depended on whether anyone was sure of what *exactly* was wrong to correct it, if there was time to fix it, and if it didn't impact other scripts. In most of the cases, it's unsatisfactory, but the conditions made it almost impossible to prevent most of the time.
Most of it comes down to simple hours in the day. With very little time to get massive amounts of footage onto tape, unless it was a major, major issue, everyone probably went with the flow and hoped that nothing too bad would get through.
Sure, there are things that seem super-stupid to us today, but remember they made it past a writing team, around 20 production staff, and the actors themselves. Human error, pure and simple.