w.t.f, why would they do that?!
not a clue.
there was short-lived thread on the PTS forum from someone whining about how Thermal Det had a lack of synergy with the rest of the Pyrotech tree, and that they wanted it to proc some kind of DoT. that thread is since buried and did not received overly positive reactions from the community, yet it seems to be the thread bioware chose to listen to.