One factor is what type of deliveries your company does. Some delivery companies want to deliver to offices; most offices have receptionists and are open 9-5 so deliveries are quick and you don't have to make multiple delivery attempts, the parking and roads are usually good, and offices tend to be in densely populated areas so you don't have to drive too long. Offices aren't open over the weekend.
Another factor is the shifts your staff are going to work. You have to give people several days a week if you want to keep them skilled and stop them needing a second job. You need managers as well as drivers. So you don't just have to hire one day's more warm bodies, you need to reorganise all your existing employees' shifts too.
If your company has always done Sunday deliveries, and your drivers knew they'd be working Sundays when they signed up, that's all well and good. But if you have a bunch of people who signed up with a company that always gave them Sunday off, and you have to change that, that's going to cost you a lot of staff goodwill, a.k.a. money.