Why in relational context to the father only and not the mother also?
Why do you think Matthew/Jesus only quotes the father and not the mother too?
There are times Mother is referenced. Historically you have Mary who was Jesus's mother.
You have Mother Church. Church spiritually nurtures, teaches, and guides believers.
Throughout life God presents through Father and Mother, situational cases.
“Honor your father and mother” (Exodus 20:12).
Ephesians 6:4 “Fathers, do not provoke your children to anger,
but bring them up in the discipline and instruction of the Lord.” Fathers are called to lead with patience and godly instruction, not harshness.
Proverbs 31:26–28 “She opens her mouth with wisdom,
and the teaching of kindness is on her tongue. Her children rise up and call her blessed…” This highlights a mother’s wisdom, kindness, and nurturing role.
Deuteronomy 6:6–7 “These words that I command you today shall be on your heart. You shall teach them diligently to your children…” Both parents are expected to teach God’s commands faithfully.
2 Corinthians 5:17 “Therefore, if anyone is in Christ, he is a new creation. The old has passed away; behold, the new has come.” when we give our life to the lord we become one with the Father.
Why we don't call God mother? There are times we refer to God in this sense, but Jesus refers to him as Father and that is currently what we have to go off of. We will know more spending eternity in heaven with him.