Perhaps because in autocracies there is no 'society' by definition? That's why they are called autocracies. Social trust is a way horizontal relationships between people are framed and organised - that is, relationship that does not go through the state. In autocracies, these just don't exist. Everything bigger than a a family (in some countries, extended family or a clan), goes through the State. There is no 'social trust' because there is no society to begin with - and state enforcement makes trust unnecessary and even dangerous.