First, because they believe they have authority to investigate the legitimacy of allegations under an archaic 2000 year old “two witnesses” policy.
Second, convictions in criminal courts bolster civil litigations which costs WT money.
Not every location mandates reporting. Where it is not they won’t. Even where reporting is mandated they do not do it with full transparency. Elders may be instructed to contact law enforcement anonymously.
Immediate reporting would benefit local elders. It does not benefit the WT organization.
Their goal is not to spend money and they use outdated theological gymnastics to justify their silence. In addition to not paying out on lawsuits, those donations keep rolling in from the publishers, because the publishers also believe the WT policy on CSA is justified.