Why? Because some statements like VACUUM cannot be used together with any other statements with single -c option. The current solution is using echo and pipe op, but it is a complication in some complex scripts - higher complication when you run psql via multiple sudo statement.
Example:
psql -c "select pg_stat_reset()" -c "vacuum full analyze" dbname
or on all db
psql -At -c "select datname from pg_databases" postgres | \