I was reading the rules and noticed it does not allow for the DLC. But this is a key feature of the game is the DLC, such as the sistermon's as well as the NX Digimon.

Why are the DLC not allowed for the Complete Edition when unlike the normal version everyone does have access to the DLC in Complete Edition

Going by definition: DLC, or downloadable content, refers to additional content gamers download for video games after their initial release.

Therefore DLC is generally not considered as part of any game for speedrunning. Even if the community were to decide to want to run with using the DLC, it would become its own category as it usually just breaks the balancing of the game similar to NG+ runs.