Change the init.ora file as given below:
# Set up Parallel Query Parameters
optimizer_percent_parallel = 50
parallel_min_percent = 50
parallel_min_servers = 8
parallel_max_servers = 10
Shutdown and Startup the instance.
Also remember that the underlying table on which you create index must be already created with parallel degree.
I have tested many times that NOLOGGING mode on table or index speeds up creation process.
The rule of parallelism is:
Parallel Query Processes = Min of number of CPUs or the number of disks on which the datafile spreads which ever is less.
Even though you have 8 CPU, but the Index data file is NOT striped, then you don't see any improvement on parallel query.