Because we've not implemented that yet!

On a related note, you can't interval partition global indexes either:

create table t (
  c1 int
);

create index i on t ( c1  ) 
  global partition by range ( c1 ) 
  interval ( 1 ) (
  partition p0 values less than ( 0 )
);

ORA-00906: missing left parenthesis

create index i on t ( c1  ) 
  global partition by range ( c1 ) (
  partition p0 values less than ( 0 ),
  partition pmax values less than ( maxvalue )
);

select index_name, partitioning_type  
from   user_part_indexes;

INDEX_NAME   PARTITIONING_TYPE   
I            RANGE    

If you'd like to see interval partitioning support for IOTs and global indexes, add a request on the Database Ideas forum:

https://community.oracle.com/community/groundbreakers/database/database-ideas