The supplies to a SEZ unit or SEZ developer are treated as zero rated supplies in the GST Law. Then why there is no specific mention in the GST Law about not charging of tax in respect of supplies from DTA unit to a SEZ unit or SEZ developer

Yes, supplies made to an SEZ unit or a SEZ developer are zero rated. The supplies made to an SEZ unit or a SEZ developer can be made in the same manner as supplies made for export:
• either on payment of IGST under claim of refund;
• or under bond or LUT without payment of any IGST.

(FAQ 5: EXPORTS)