The reason why aldehydes are more reactive for nucleophillic additions is because ketones have 2 alkyl groups while aldehydes only have. Sterics play a minor role, however. The reason why aldehydes are more reactive is because there is more positive charge on the carbonyl carbon. Ketones, with 2 electron donating groups, have less positive charge on the carbonyl carbon, and are thus less reactive to nucleophillic attack at the carbonyl carbon. Keep in mind that carbonyls are sp2 hybridized and thus planar making sterics less of an issue.