Why does ontogeny recapitulate phylogeny?

Because any change early in development affects many later events, it tends to be very disruptive --just like moving the lowest blocks in a Lego structure is more likely to make it fall down than removing a top one. This means that the earlier in development a mutation expresses its effects, the lower the chances that it will endure or get fixed in the population. Thus, the frequency of mutations affecting early development is lower than that of those expressed later on, and thus the earlier in development, the more similar different species are to each other and to their original ancestors. This explains why ontogeny (development) recapitulates phylogeny (evolution).

Corollary: When available, fetal or newborn traits, and not adult ones, ought to be used to classify phylogenetic relationships when genomic information is not available.