I've lost count of how many times I asked that very question.
Usually the response is "to protect themselves from liability" or somesuch, but that makes me scratch my head almost as much. After all, wouldn't documentary evidence that they knew about the problem but concealed it cause even greater liability if it were to fall into "the wrong hands"?
It's also been suggested that because the higher-ups were full-on True Believers, they really did think that Armageddon would drop any day now, and therefore, said record would never be scrutinized by "Satan's World"... not to mention that truly believing they were following "God's Word" in the matter (particularly with regards to the two-witness rule) would also reinforce the belief that if they "stayed the course" just a little bit longer, it would sort itself out.
(There's also the odd historical phenomenon that authoritarian regimes can't seem to help but keep meticulous records of everything they do, no matter how awful.)
The closest I've personally come to a plausible theory is that the Org started keeping records of all the shitty things its members did, in order to ensure that unqualified brothers wouldn't be given congregational privileges and then f**k up and embarrass the Org (wow, that worked so well )...
... but because of the prevalence and institutionalized nature of child abuse in the Org, that particular data subset ended up evolving into its own thing (the current database), a step removed from records of the usual disfellowshipping offenses like adultery, gambling, etc.
They had to keep recording instances of abuse in the Org in order to try and leash these assholes, but there were so many increasing reports and poorly handled cases (not to mention outdated social views that exacerbated rather than mitigated the problem), that it couldn't help but get away from them.
By the time they realized how bad it actually was - partly thanks to Barb Anderson, ironically - they were too locked into the methodology. Reforming it at that stage would be acknowledging that their beliefs and policies had made the problem worse rather than fixing it, not to mention that the kind of reforms needed to fix it would have been too radical and liberal for the Org to comfortably implement.
Completely unacceptable for "God's Earthly Organization" (not to mention that change naturally comes very slowly to ultraconservative groups).
Add internal resistance from higher-ups who themselves may have been guilty of past offenses...
...and it all led up to the current legal catch-22... screwed if they stopped it, humiliated if they if they released it, buggered if they didn't, and legally f**ked if they got rid of it.
The Org really does seem to have a knack for "Biblically-based" actions that come back later and bite them on the ass.