Oracle XE Backup Restore Problem
After datafiles are restored, oracle performs complete recovery by applying all changes from archived and online redolog files. Those changes also covers "drop statement" you did run after the backup. Hence, the recovery after the restore applies "drop table" statement during "restore database"

In order to recover drop table stament, you need to choose/perform incomplete (time/SCN based) recovery. That will bring database to a time before the drop statement is run.