Not only for the reasons Dakota said, but also because it's a burden on him.

When someone has any long-lasting issue, it's frequently easier to find a person to talk to that you don't see in a social or other environment.

If I ever needed to talk to someone about something deep that wasn't likely to be solved quickly, I would want to talk to an "independent outsider," not someone I already had a relationship with or someone who was my general spiritual adviser. To use my pastor for these things would change the relationship and put an undue burden on him.

Now, if it were anything "mundane" like an Internet addition, I might ask my paster for a referral to a professional counselor, or to another pastor that I didn't see in a social setting.

Of course, in today's climate, unless you are a died-in-the-wool confirmed celibate who has never violated the law and never will, the only three people you can really open up to about pedophilia are:
1) your lawyer, but only about past deeds
2) the priest you are confessing your sins to, but only if your church requires it (Roman Catholic, a few others)
3) your spouse, but only if you know she won't tell others, but even then there are limits

Anyone else is likely to be covered by "mandatory reporting" laws if you talk about things you've done illegally in the past or things you might do in the future. Even your lawyer, your priest, and maybe your spouse are required to report you for illegal acts they believe you will do in the future.

By the way, in some states, as officers of the court, lawyers must report any planned illegal activity, not just the usual mandatory-reporting-law child-sex items.