Some people can't handle the pressure of this type of work. Sales reps need excellent product training and sales training. A rep needs to understand how your business operates and how to sell your products or services. Bad support from the company. In some companies, the reps do a good job, but the company fails.
Processes are slow and inefficient.
Customer service people don't support them well. Shipments are slow and inaccurate. The list goes on and on. Even a good sales rep can fail if the company fails them. There are some of the key reasons why sales reps fail.