The problem you have with light isotopes is that centrifuge...

The problem you have with light isotopes is that centrifuge technologies (and also slightly different boiling points) become far more attractive than with higher atomic mass elements. For example Lithium has two isotopes ⁶Li and ⁷Li. The effectiveness of centrifuge technologies is that they are more efficient with high mass ratios. For example for Li the mass ratio is 7/6=1,166667 but for uranium it's 238/235=1.0127659. So for light elements the mass ratio helps centrifuge technologies but this disappears for high atomic numbers (where centrifuge becomes less attractive).

The key to lasers is that molecules tend to cluster (in twos or more) by a property called Van De Waals forces (this is what is called a lower energy state) for example nitrogen (N) hangs around normally as N₂ this applies to most gases (e.g O₂, H₂ etc). The molecular weight of UF6 is U (235)+19 (fluorine) x 6 = 349 and for ²³⁸UF₆ is 352 thus the mass ratio is 352/349=1.00859 (i.e very small so centrifuge is not very effective). However they normally go around as dimers (2 x UF₆). What the laser does is to cause the ²³⁵UF₆/²³⁸UF₆ dimers only to become excited and (only those dimers) to disassociate into monomers (individual molecules). Now the mass ratio is 349/(2*352)=2.01719 - that makes it easy to separate by sending the gas around a tight corner. I have oversimplified this since you mix the UF₆ with a "carrier" gas so as to improve the sensitivity. What I am trying to explain is that laser technologies lose their advantages with lower atomic weights such as Lithium, Hydrogen, Oxygen etc and it's not worth having the cost of the laser (i.e centrifuge is effective and simple)