u should ask this question from a qualified scholar who will give u a plausible reason.
In my humble opinion a possible reason may be the fact if prophet had appointed someone as his successor ppl may start following that person and his descendent onwards as a source and authority for deen’s interpretation.
Such kind of behavior could be detrimental to the spirit of Islam. The basic source of Islam is always quran and hadith and for practical guidance we have to follow sunnah .
sahaba were the persons (except for prophets) this world have ever seen and may be prophet had thought that they were competent enough (surely they were) to choose the best person among them to be their caliph.
(if u want to know how Allah has praised sahaba read the translation of sura fatah).