I can't be entirely sure, but my guess is that it's just a too much of a different part of speech to be considered a conjugated verb. Participles often function as adjectives or adverbs (https://en.wikipedia.org/wiki/Participle): "No está cocinada." -> "describes" whether or not the meat is in a state fit for consumption.
Conversely, conjugations are just different forms of verbs, adapted for the tense (e.g.: past, present, future) and the entity that performs the verb. Unlike this, as far as I know, a participle has only the one form for all such possible cases, which is also probably why it is not listed as a conjugation.