Why cannot I create triggers on objects owned by SYS?

While trying to create a trigger named ghazal_current_bef_upd_row :

create trigger ghazal_current_bef_upd_row
before update on ghazal_current
for each row 
when (new.Rating < old.Rating)
begin

insert into ghazal_current_audit
 (GhazalName,Old_Rating,New_Rating)
 values
 (:old.GhazalName,:old.Rating,:new.Rating);
end;

I get the following error :

Error report:
ORA-04089: cannot create triggers on objects owned by SYS
04089. 00000 -  "cannot create triggers on objects owned by SYS"
*Cause:    An attempt was made to create a trigger on an object owned by SYS.
*Action:   Do not create triggers on objects owned by SYS.

Both the tables named ghazals_current and ghazal_current_audit were created by SYS. Why cannot I create a trigger on the table created by SYS .