Because there is not enough information available to give an adequate assessment of the embryo quality and therefore to choose the best embryo. On the 2nd day, the embryo only has 4 cells, so it does not provide sufficient information on the rates of division or embryonic morphology, as it may happen that initially the embryos look very good but alterations appear later.
On the other hand, on the 4th day there is sufficient information about their development until the 3rd day, but as it has only begun to compact (forming the morula) it is not known whether it will give rise to a suitable blastocyst, which can be assessed on the 5th day.
Therefore, it is more appropriate to monitor embryonic development until day 3, when the embryo has 8 cells, or until day 5, when the embryo is already in the blastocyst stage.