H3PO4 Is Tribasic While H3PO3 Is Dibasic , Why?

Basically it because  of the structure.
H3PO4 is
  O-H
  /
O=P-O-H
      \
  O-H
so each H+ can be removed to leave -Ominus.
H3PO3 is
  O-H
      /
O=P-H
     \
  O-H
Pminus is just too unstable compared with Ominus. Oxygen is more electronegative, but I reacon its more complex than that.