Though it's impossible to know the exact events that lead to the evolution of human intelligence (evidence like fossils don't really leave us many clues in this area), one could speculate that it was our specific physiology that lead to our becoming more intelligent, probably in combination with environmental factors such as climate change (intelligence can be an important key to adaptation).

There are other theories too, such as that there is a 'critical mass' of neurons which must be acheived in order for an animal to reach a human-like level of consciousness. If that's the case, then other mammals simply haven't had an evolutionary need to meet this critical mass, but that doesn't mean that the potential isn't there.