Because there are no rests in that music, Dorico is interpreting the whole thing as a single phrase, and Dorico will not change its mind about the maximum possible condensation midway through a phrase. The differing rhythms at the end of the phrase therefore prevent unison on a single stem at the start of the phrase. In a future update you will be able to create a condensation change at the point at which the rhythms begin to differ, which will give Dorico a clue as to where you consider the phrase to begin, and that will allow Dorico to produce unison condensation for the preceding bars.