We can not answer such questions. With matters like this scriptures are our sole guide. So, we can not ask why a particular deity is not there in the Dashamahavidya list.
The lists found in scriptures do not include Bala Tripurasunadri that's why she isn't included. This will be the only answer.
Brihaddharma Purana, Madyabhaga gives the usual list:
MahAvidyA imAh proktA nAmAnyAsantu varnaye |
KAli tArA mahAvidyA
shodashi bhuvaneswari ||
BhairvaichinnamstA cha sundari
dhumAvati cha mAtangi mahAvidyA dashaiva TAh ||
Brihaddharma PurAna, MadhyabhAga.
This list gives the names of KAli, TArA, Shodashi, Bhuvaneswari etc but does not include BAlA.
The MundamAlA Tantram gives the same names and does not include BAlA too.
KAli tArA mahAvidyA shodashi bhuvaneswari |
chinnamstA cha vidyA dhumAvati tathA ||
mAtangi kamalAtmikA |
EtA dasha mahAvidyA siddhavidyA
But still BAlA is a Supreme Goddess because she is one of the three Tripura Devis.
Lord Shiva says:
TripurA trividhA devi bAlA proktA purA shive |
devi nityA-tantre mayoditA ||
IdAnim sundarim devim srinu pArvati
Tripura Devis are of three types; Among whom I have already said about
BAlA and Tripura Bhairavi in NityA-Tantram previously. Now, listen O
PArvati with due respect about Tripura Sundari.
Sri MAtrikA bheda Tantram 7.1,2
One meaning of Tripura is the Devi who existed before the Trinities who are in turn the Supreme deities of Hinduism.
Therefore, she is a Supreme Goddess even though she is not included among the ten MahAvidyAs.