CLOBs in a multi byte character set data type allocate a minimum of 2-bytes storage, even if the character being stored is single-byte. This will significantly inflate the storage requirement and has other performance implications such as: enabled storage in row will only allow just under 2000 characters in its 4000 byte limit.
The introduction of Extended data types does not have the same restriction. A VARCHAR2(32767) will hold just under 4000 (single byte) characters in the row before switching to LOB segment which shows that there should be no technical barrier to implementing this change.
It's not unusual to need to store character strings that are normally in the range 2000-4000 bytes but occasionally too large for an extended data type, so a storage-optimised CLOB would be ideal.