As I understand, abuse filter checks each edit against all available active filters in ascending order with respect to their assigned number. For example an arbitrary edit will be checked against AF number 1, 2, 3, and so on.

Filters are usually created in order, too.

But some filters that are created later, have higher priority. I can think of two situations that this could happen:

1. AF n+1 is set to disallow an edit for IP addresses and AF n is set to warn all users for that edit. It makes sense to put AF n+1 before AF n, so that if an IP address tries submitting that edit, they will see the disallowed message first and won't try again...
2. Some abuse filters perform more critical tasks and we don't want them to be ignored because of reaching the condition limit.

If moving filters was possible, we could rearrange such filters and avoid the above issues.

Of-course this can be done manually, but there are several problems with manual copy-n-pasting:

1. It's boring! (you'll have to copy all the fields, recheck the checkboxes..., set warning message, etc...)
2. Their edit history is not preserved.
3. After renaming a filter and overwriting it as described above, all the logs of the older filter will appear under the new name. This can easily lead to confusion...