Why are there no standard potentials more negative than-3V, andmore positive than about +2V, in aqueous solution?

First of all we should know that this value of standard potential is not an absolute value but it is relative to the hydrogen electrode(whose standard potential is assumed to be 0V). Then reduction potential value with respect to the hydrogen electrode was obtained for other elements and the values got was totally experimental.
we can understand about very low value of reduction potential ofLi (highest with -ve sign)as of high tendency to loose electron as well as extremely high hydration enthalpy.
Li....+ e-........>Li-
Similarly for F high positive value is due to high tendency to get electron and high hydration enthalpy.
F....+ e-........>F-
So it depends upon ionisation energy(for metals), electronegative and since it is measured in solution(generally water so hydration energy, lattice energy) and after all these we get particular value for potential.
swmaller the ion higher the hydration enthalpy................