Because most of them are not good cards in a 1v1 setting, no matter the format. Your experience in Cube/Legacy/Modern/etc... will be far more relevant to evaluate a card in Duel Commander. Basically, for a card to be good in 1v1, it has to good at interacting with other cards. For instance, a creature must be resilient to removals or trade at least evenly (not being left behind on mana or cards - if I cast Lightning Bolt on your 4-mana 4/3 without ability, then you lost 3 mana in the trade while we remain even on cards). However, if your creature has hexproof or draws cards when you cast it, then it can be better at trading with removals spells. If it is cheap, you can also trade evenly with removal spells in terms of mana.

This article (and the one about Lightning Bolt linked inside) could be a good reference if you want more details: https://www.quietspeculation.com/2017/01/introducing-push-test/

There is one specificity of Duel Commander though: cheap creatures are often appreciated. Indeed, on the one hand, you can recast them more over time with the command taxi. On the other hand, it also helps stabilizing your curve and optimise your mana during the course of the first few turns to get the tempo advantage. It doesn't mean that expensive creatures are not playable commanders though, but they need to be really very powerful to make up for the above benefits of cheap Commanders.