Answer
Companies encourage their employees to hold large amounts of stock of the company to have more pride in the company (and how the company does), rather than simply the employee's own pay.
Work Step by Step
Employees might not want to hold stock in their employer if the industry is not doing well (so that the company is not doing well). Also, the employee is taking too much risk by depending on one company for so much of their financial wellbeing (paycheck and stock price, which might affect their retirement accounts).
As an example, in the financial crisis in 2008, many bank employees were encouraged to hold their employer's stock. When some banks closed, those employees lost their jobs and took a huge loss on the sale of the company's stock.
