First, it is more accurate to say Zakah is 5% for money that is earned by interaction of capital and labour, and is 10% for money that is earned by only one of these factors.
Second I don't think we can make a general rule that businessmen make more money than professionals. Only those businessmen earn a lot who have been successful in their business and have managed to make it vast and stable. Not many businessmen are like this, in fact not many businesses survive enough to bring any real profit to their owners.
It seems like the logic behind the rule, as mentioned above, is not related to the amount of earning but relates to the risk that might be involved. In general and overall, when an earning is based on both capital and labour, the risk of loss is higher compared to the situation where only capital or only labour is needed.
As you know there is also 2.5% Zakah due on any wealth that is not in personal or business use.