Logical reason is to avoid scammers convert from non-self-moderated threads to self-moderated ones when their scam threads are detected by community. Then the conversion will help scammers easily to silent the community by deleting proofs in other posts.

Forget to choose self-moderated option? Solution is there: Move it to archival, rename the thread, and create a new thread with self-moderated option (only helpful in first minute or hours after you create threads).