Why do the negative oxidation states not go all the way down to -8?
Let's think about what we would need for this to occur - i.e. an oxidation state of -8.
We would need a central atom that has considerable electronegativity - one that is at least more electronegative than its attached atoms. This already excludes numerous molecules; most molecules' central atoms are, on a relative basis, more electropositive rather than electronegative.
In addition, this atom would need to have one of the following configurations; "allocated" refers to the assignment of oxidation state - since oxidation states are assigned on the premise that all bonding is ionic - the more electronegative element in a bonding configuration is "allocated" all the bonding electrons in the calculation of oxidation states. I.e. in H2O, the oxygen would be allocated both pairs of bonding electrons in the H-O bonds, and oxygen would then "have" 8 electrons, and its oxidation state would be -2.
1 valence electron and be "allocated" 9 electrons
2 valence electrons and be "allocated" 10 electrons
3 valence electrons and be "allocated" 11 electrons,
and on the other extreme:
6 valence electrons and be "allocated" 14 electrons,
7 valence electrons and be "allocated" 15 electrons,
8 valence electrons and be "allocated" 16 electrons.
As we can see, the first set of criteria are basically incompatible. Elements only having 1 or 2 valence electrons are going to be small (on a relative basis). These elements would also much rather be oxidized than reduced. Oxidation allows these elements to easily become isoelectronic with a noble gas.
The other set of criteria for a negative 8 oxidation state is also hard to fulfill. None of the group 6 elements except the possibility of sulfur and elements below it are hypervalent - i.e. they can accommodate more than 8 valence electrons. Even if we consider sulfur as able to accommodate 12 valence electrons (as some depictions of sulfuric acid suggest) we must remember that sulfur is not very electronegative and unlikely to be allocated any electrons.