I tried to buy XYZ stock from a large online brokerage company, and they wouldn't place the order. Why?

I tried to buy XYZ stock from a large online brokerage company, and they wouldn't place the order. The rep said the brokerage has a list of companies that they will not place orders with, but the rep couldn't tell me why. They have certainly allowed me to buy stock of other companies in the past that have subsequently performed poorly, so I ume that's not the reason. So why would they do this? I emailed the company whose stock I want to buy, but no response.