Why is voltage higher at positive wire instead of negative wire in this example?
An unfortunate convention. 

Conventional Current assumes that current flows out of the positive terminal of a source.

Electron Flow is what actually happens and electrons flow out of the negative terminal of a source

When Benjamin Franklin was writing up his discovery, at the time without any detailed knowledge as to how it was actually occurring, he had a choice what is to be labeled positive and what is to be labeled negative. 



image credit http://xkcd.com/567/