A. The most obvious answer: because you can. Marriage has its own gravitational force -- in time, every task that needs doing cleaves to the partner who's most adept at it.
If your husband works full-time, out of the house, and you don't, then that's a practical reason you've been elected Keeper of Details. He's not around as much, so it makes sense that your brain is the family database. If his were, you'd have to call him at work to check on what time swimming starts, which could get annoying.
But if you both work similar hours, you're certainly justified in wanting to distribute the parenting brain-load more equitably. So sit down and divvy things up. You're in charge of clothes and shoes; he's in charge of school-project supplies. You book the sitter; he books the checkups, and so on. Be sure to consider what other minutiae your spouse keeps track of that may go unnoticed. If he takes care of car inspections and maintenance, that's a pretty big something.