One reason given in the suttas is that he did it by request, and out of compassion.
As the Blessed One reflected thus, his mind inclined to dwelling at ease, not to teaching the Dhamma.
Then Brahma Sahampati [etc...]
Then the Blessed One, having understood Brahma's invitation, out of compassion for beings, surveyed the world with the eye of an Awakened One.
The Buddha's not viewing the world as 'self' wouldn't stop him from having compassion.
Compassion is one of the four recommended attitudes to have towards others.
Also, views such as,
There is nothing given, nothing offered, nothing sacrificed. There is no fruit or result of good or bad actions. There is no this world, no next world, no mother, no father, no spontaneously reborn beings; no contemplatives or brahmans who, faring rightly & practicing rightly, proclaim this world & the next after having directly known & realized it for themselves.
... are defined as "wrong view"; I guess that would be some kind of "nihilism".
