I’m not sure I find the premise in the OP persuasive. There has always been a stronger incentive to provide a vigorous defense in the case of a sex offender, because of the widespread belief (true or false) on the outside that sex offenders are treated especially poorly in prison.
If I had to make a conjecture, I would say that the dropoff in the mid-90s could probably be explained by changes in some elective parts of the process that recognize Megan’s Law. Guesses:
- Prosecutors or grand juries are less willing to proceed on weaker sex offense cases because of the stigma of sex offender registration
- Defendants are refusing plea bargains that require registration
Note that both of these affect marginal cases where conviction with registration would be more difficult to begin with. In either case, the conclusion would not be that Megan’s Law causes dangerous offenders to be acquitted at higher rates but that prior to the the law there was an insufficient distinction between threat and non-threat offenders.