Why, because the evidence showed he was guilty. His defense was actually based mostly on a technicality over admissable evidence, not over the truth.
Given the fact that another man was later convicted of the crimes Dreyfuss falsely was convicted of and spend 4 years on Devil's Island because of, I fail to see the technicalities. He was convicted because he was Jew and from Alsace and when the read spy Major Esterhazy was arrested 4 years later the military tried to cover up because it was an embarrament that they had convicted a man without any real evidence, but mainly because he was suspicious qua his religion and origin. Hence the classic article by Emile Zola:
"J'accuse"
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