תַּנָּא דְבֵי מְנַשֶּׁה: ״עַל יָדְךָ״ — זוֹ קִיבּוֹרֶת, ״בֵּין עֵינֶיךָ״ — זוֹ קׇדְקֹד. הֵיכָא? אָמְרִי דְּבֵי רַבִּי יַנַּאי: מְקוֹם שֶׁמּוֹחוֹ שֶׁל תִּינוֹק רוֹפֵס.

The Gemara comments: The school of Menashe taught the following. The verse states: “And you shall bind them for a sign on your arm, and they shall be as frontlets between your eyes” (Deuteronomy 6:8). “On your arm,” this is the biceps muscle of the arm; “between your eyes,” this is the crown of the head. The Gemara asks: Where exactly on the crown of the head? The school of Rabbi Yannai say: Phylacteries are placed on the spot where a baby’s head is soft after birth.

Is there a possible deeper meaning to the tefilin being in the soft spot, that then becomes hard?

כד הקמח א:ט

Kad Kemach says the tefilin sit lower on the head than the top of the forehead to hint at a humble lower place.

 

חתם סופר אה״ע א:צ:ז

בוודאי בלשון תורה הוה בין עיני' במקום שרש העינים מקום שער שמוחו של תינוק רופס ששם עורקי העיני' שופכים מהמוח אל העיני'

Chasam Sofer notes that indeed the Tefilin are really worn “between the eyes״ because the optic nerve leads to the visual cortex which is located above the tefilin. That is, what the eyes see are actually processed in that part of the brain,

I wonder, if there is another idea being hinted at by this location. This part of the head, the Fontanelle, is originally soft to allow for the baby’s head to pass through the birth canal. Within 2-18 months the six bones knit together. Intellectual thought requires transition and development. One cannot be too hard headed if he wants to grow and pass from one stage in life to the next. So while the tefilin on the head remind us of our intellectual duties and devotion to G-d, we must also keep in mind that humans need flexibility to grow and develop.

In addition, the last point made by the Chasam Sofer, brings to mind some of the research regarding emotional stimuli, the visual cortex, and Trauma.

It seems that there are multiple paths from your optic nerve to the brain.  When something is not emotionally stimulating, or not seen as an immediate threat, what is seen goes through the visual cortex and is processed.  The visual cortex is the primary cortical region of the brain that receives, integrates, and processes visual information relayed from the retinas. However, if something is emotionally stimulating, possibly reminding someone of a past trauma or danger, the brain takes a shortcut and goes straight to the amygdala.

This can easily be understood from a survival mechanism point of view. That is, if someone sees a crooked brown object out of the corner of their eye in the woods, there is little time to discern whether it is a snake or just a stick. Therefore, in this case, the amygdala goes into overdrive and skips visual processing to allow anyone to react more quickly to a possible danger. So the person will jump up rapidly to get  away from the snake, even if it in the end it doesn't turn out to be a snake. On the other hand, if there's less emotional stimulation and less threat perceived, there is a bit more time to analyze what is being seen before your eyes and it goes through the visual cortex.

This is important in understanding trauma, because often people are triggered by things that they perceive as threats due to past experiences that lead to an immediate mental shortcut without having conscious introspection. It is useful to know this and to work on developing mindfulness in order to discern between friend or foe, snake or stick. This can lead to less contentiousness in relationships with people. 

Perhaps the presence of the Tefilin Shel Rosh over the visual cortex as opposed to be located directly between the eyes is to remind us and encourage us to note this distinction, To understand that what we see  with our eyes is not the full picture. 

Source: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5222876/