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