Solos, generally, don't have the steady volume of work to merge into a partnership.
Some partnerships are little more than marketing ventures.
Actually, its not so much that that solos make below average incomes, its that the average income for a solo is much less than the average income of an attorney of a big firm. The partners at firms are those who generate business. The solos who make money are those who generate a lot of quality business. I'm not sure what income you would consider good. Please give me your thoughts and success stories.