The implication here is that we have a medical doctor who came later than appropriate. Too late implies a dead or injured patient as the result of the delay or that the event or reason for the visit by the doctor is over or concluded; so late implies he came later than he should have come or later than he was expected, which is I believe the correct meaning of "for sent."
Duolingo will have to confirm or correct as the case may be for the correct meaning of "for sent" as used here.
However the bigger issue is that "doktor " på dansk based on prior Posts and my ord bog is a PhD not a medical doctor. As such, if we are discussing the late arrival of a medical doctor the correct word is lægen; "hvorfor kommer lægen for sent." If we mean why did the PhD doctor come to the event so late, then that is the correct translation.