Why my Washington DC tax return doesn't include my wife and dependents? Note that I am filling only D40B (non-resident request for refund).

Washington DC does not tax non-residents.  Therefore, if you are filing that form, it is because DC tax was mistakenly withheld from your paycheck and you are requesting a refund of those withholdings because you will be paying taxes to your resident state instead.  Having your spouse and children on that form does not change the result, since the refund is based on the income you earned in DC.  (If your spouse also had DC tax mistakenly withheld, she would file a separate form to claim that).

If the return says $0, however, then you do not need that form and you may delete the DC nonresident return.