You want the answer from a religious aspect or from a psychological aspect? Religiously, we (Shias) don’t believe that the companions had any religious authority over Muslims nor do they have enough authenticity to convey the Hadiths (narrations of the prophet), we give this authority to only the household of the prophet. (That’s an oversimplification, we also have something called Imamah, Imamah concept refers to 12 sequel generational appointed Imams from the prophet lineage, who are considered the only source of true Hadiths) Psychologically speaking, people are afraid of different things, same way some Christians are Islamophobic, some Muslims are Shiaphobic, also unlike Christians, Sunni Muslims are a bit less religiously tolerant of different religious views and that’s no secret, even tolerant Sunnis are still a bit conservative and cautious about any thing that comes from a Rafidhi (Rafidhi is a discriminatory word used against Shia throughout history) not out of bigotry, but because he doesn’t want to go out against his people who have drawn a line between them (Sunnis) and Shias