Do we honestly only have two innate fears? I would have thought that most infants probably don't love spiders or heights. Maybe I'm wrong on that, but at any rate those seemed conditioned genetically and not from past experience.
Heights: fairly obvious evolutionary advantage to stop people falling off cliffs and the like
Spiders: I would assume that this is a reflex against anything "weird/unnatural".
If babies aren't scared of these, then maybe it's because they haven't yet gathered enough information to be able to distinguish strange from familiar? There must be a consensus on this somewhere.
Edit: didn't realise one of the two fears was falling. Replace everything about heights with "the dark" and "being viciously eaten by predators you couldn't see". Fear of the dark certainly has an evolutionary pragmatism to it, if babies aren't scared of it I can only assume that they need to develop more for the trait to display itself.