Explain why detritivores, decomposers and omnivores are not assigned trophic level

They can be found in several trophic levels, as they are related with several trophic level, hence, they cannot be placed on a specific trophic level.

Explanation:

A trophic level is a specific position occupied by a group of organisms on a good chain in relation to their nutrition.

Dentrivores feed on the waste of other organisms and cannot be really placed on a specific trophic level, likewise decomposers, of which they feed and decompose the body of organisms.

Omnivores feed on both plants and animals, which includes producers and consumers. Placing omnivores on a specific trophic level would not be really straightforward also.