Because the official evidence is held by the incumbent government. The evidence we do have access to, from extensive exit polls by neutral auditors to the mandated voting station slips (small pieces of paper that each voter is issued giving them the electronic count so far at that station) both track a 60 - 70% lead for the opposition.
In response, instead of releasing the official report, the incumbent government has brutally crushed several protests (even killing protestors), arrested the opposition, and claimed victory. Not exactly the pattern of behavior of an innocent victor.