We have found the explanation from Oracle's doc. Here it is:

Importing a Snapshot into a Different Schema
Snapshots, snapshot logs, and related items are exported with the schema name explicitly given in the DDL statements; therefore, snapshots and their related items cannot be imported into a different schema.

If you attempt to use FROMUSER and TOUSER to import snapshot data, an error will be written to the Import log file and the items will not be imported.