Evil wizards - and in some cases just evil characters generally - are more powerful if doing evil provides rewards that are greater than the rewards for doing good. Often this takes the form of specific actions that only evil people would conduct that provide a material benefit with not counterbalancing good action.

A simple and classical example is human sacrifice: if killing people provides power, and saving people doesn't provide the same amount of power (or no power at all) then evil is more powerful and your world has an inducement to terrifying cosmic horror that will produce a world of hideous grimdark. In D&D context the type example is Dark Sun: Defilier magic allows for power at the expense of the ecosystem. The result is a destroyed ecosystem and a grimdark hellscape ruled by corrupt wizard gods.

It should be noted that this is bad setting and system design if you are intending to produce a world that is not a grimdark hellscape or a rumination on cosmic horror you should not build a world this way. In fact, given incentives, laziness, and other factors it is generally better design if good is explicitly stronger than evil, though usually a more difficult path. Many settings often fail to do this. One reason is the influence of salvationist moral thinking in which the entire world is functionally a test. In a salvationist scenario it doesn't matter if the world is a grimdark hellscape, only personal virtue matters, because everyone is facing an inescapable, all-powerful judgment in due course and will be rewarded appropriately. Many settings apply options that make sense in the context of a salvationist world - like the ability to sell your soul for temporary power - that become meaningless when judgment will never come.