According to this write up of the development of the various forms of Kaddish, the Kaddish began in Second Temple times as what we now call the "half-Kaddish." Then a section was added for the Rabbis (kaddish Derobonon) and later,
By Talmudic times, it became customary to conclude the prayer service with the Kaddish. A sentence was added (the line beginning titkabel, "let be accepted") that replaces the passage for the rabbis and disciples and asks God to accept all prayers that were recited. This Kaddish is called Kaddish Shalem (Whole or Full Kaddish) and is still said by the chazzan at the end of the service. The full Kaddish includes two sentences, added to the Half Kaddish around the eighth century, that reflect the traditional yearning for peace (Yehei shlomo rabba and Oseh shalom).
Later there is record of the Orphan's Kaddish:
The first mention of mourners saying Kaddish at the end of the service is in a thirteenth century halakhic writing called the Or Zarua. The Kaddish at the end of the service became designated as Kaddish Yatom or Mourner's Kaddish (literally, "Orphan's Kaddish").
In the creation of this form, a line is removed -- the line beginning Titkabel.
What is it about this line that makes it inappropriate for the Orphan's Kaddish? Was the removal simply to differentiate the two forms?