They agree to fight on most similar issues facing America. Both have great leadership qualities. They together care about the American people and wont be bought by Wall Street.
The point of a VP is to appeal to a broader voting base to rally support for your side. Warren is very well liked by hard leftists but she and Sanders already have a ton in common in terms of stances on the issues, meaning she wouldnt be bringing in more people to his side. Davis on the other hand is different enough from Sanders on the issues to get more people to his side.