Because you can't really use those feats as an accurate standard of who is most powerful. If Sidious had the Helm could he have done the same? Probably better. You have to measure them based on raw talent and power, and judge them based on what they can do without external items to boost them. If you are going to use artificially enhanced feats like that then Jerec would easily be the most powerful after he connected with the power trapped in the Valley of the Jedi....but that doesn't tell us how powerful Jerec was....it tells us more about the power trapped within the Valley of the Jedi than it does about Jerec because when you look at Jerec he wasn't really that impressive at all. Taking force users and gauging their power at the base levels is going to give you a much more accurate look at individual power levels. When you do that you see fairly quickly that in terms of feats accomplished without the use of external boosters Sidious > Odion > Jerec, and you can see that because you are looking at each of them on a level playing field.