Because of Sonni's own dacuments, which prove that Imam Ali should be khalif and Imam emidiatly after the holy Profet.
You can see the book "Al-Morajeat"
المراجعات
this topic is the one that Shia dose not have any especial idea except what Islam says. when we refer to Islamic teaching we figure out that Islam accept just one way for people to Rescue in the other world, due to that when Quran uses word"Noor" uses it in singular form, but when uses word"Zolmat" uses it in plural form