Why? What caused some to evolve but not others. And what is the scientific proof for why this happened?
This occurred during a time of changing conditions in Africa. Some 6 million years ago, the forests were shrinking and the grasslands expanding. That put selection pressure on the ape-like critters living nearest the edges of the forests; some had to adapt to the grasslands. The critters in the main forest areas were doing just fine and needed to change little.
Given time, and continued shrinkage of the forests, the groups along the edges needed to adapt to the grasslands or die out. At least one group, from which we are descended, did adapt.
So, the groups that remained in the forest evolved slowly, becoming the apes we know today. The groups at the edges were forced to adapt to new conditions and eventually, after many adventures, became modern humans.
By the way, science does not deal in proof, but in evidence. This is what the evidence currently suggests. If more evidence is found which suggests some changes in current theories, science will make those changes. This is the exact opposite of religion, which adheres to dogma in spite of evidence to the contrary.
If you have any other questions about this let me know. I studied it pretty intensely in graduate school and still remember a few of the details.