That is a good question. The Greek successor government made a number of mistakes between the time of his death and the German invasion, most importantly leaving the bulk of the Greek army facing the Italians as the Germans prepared to invade. That's understandable from the point of view of national pride. The Greeks didn't want the Italians advancing against weakened Greek defenses and claiming victory. At the same time, as a realistic defensive strategy it was awful. It let the Germans coast to victory.
The Greeks were not going to stand up against the Germans in the long run in any case, and they may well have known that, in which case what they did was somewhat rational. On the other hand, the Greeks could potentially have gained significantly from prolonging the defense and putting troops in a position to be evacuated. Given the tight time frame the Germans had for Barbarossa, a better Greek defense against the Germans might have meant more Greeks got away to fight in Crete, which could easily have turned the tide there.