He got elected democratically. In the German system, one party gaining the absolute majority is very rare and coalition governments are the norm. The cancellor is usually the candidate of the party with the largest percentage in that coalition. In the case of the NSDAP, that was Hitler. The conservatives formed a coaltion with him because they were sure the conservative president would be able to keep Hitler in check. Unfortunately, said president did the opposite and was instrumental in giving Hitler absolute power.