Maybe they've got 99 others going and they need 100 to get a group discount.

Maybe someone is keeping statistics on field trips and the school wants to boost their numbers.

Maybe they'll get a little more money if your kid goes on the trip (and is on the school clock).

Maybe if they make it optional for anyone, then they have to make it optional for everyone, and they don't want to do that for some reason.

I have absolutely no idea. It's very bizarre that they're so secretive about their real reasons.