Can any one please explain Why joiner is not used after Sequence generator or Update statergy?
It's because joiner may render useless the logic achieved until those two transformation.
Joiner after sequence generator - Purpose of a sequence generator is to generate a new or missing key value. Depending on the join type, you may drop this key value.
Joiner after update strategy - Purpose of update strategy is to determine how a record needs to be processed into the database - insert/update/delete. This decision generally gets made when all the data has been pulled in and every logic/transformation has been applied. Depending on the join type you might miss essential changes to your database.