Norms changed over the time period in question greatly reducing the need for physical violence or immediate threat thereof to prosecute someone for rape or a rape adjacent crime.
Marital sexual assault was generally prosecuted under domestic violence laws (which themselves became broader and stricter over time) on an as-needed basis but the required fact pattern for prosecution in 1965 or so was a higher bar in general than the standard set by the laws by 1990 or so. Furthermore, plea bargains rose in popularity over the same time period and DV went from a non-issue misdemeanor to a serious problem on your record over the same time period so examining conviction stats for various crimes is difficult.
>I assumed plea bargains were always this popular. Any idea why they rose in popularity during this specific period?
A variety of reasons but mandatory minimums and stiffer sentencing in general (both made popular by the war on drugs) are the bulk of the reason.
When your lawyer could say in confidence that based on the fact pattern you're all but assured sentence on the order of a lesser crime that more correctly fits the fact pattern you're more likely to go to court and try and clear your name. When you're guaranteed a minimum sentence should you lose taking the bargain seems more reasonable.
Marital sexual assault was generally prosecuted under domestic violence laws (which themselves became broader and stricter over time) on an as-needed basis but the required fact pattern for prosecution in 1965 or so was a higher bar in general than the standard set by the laws by 1990 or so. Furthermore, plea bargains rose in popularity over the same time period and DV went from a non-issue misdemeanor to a serious problem on your record over the same time period so examining conviction stats for various crimes is difficult.