Why do some breeders let pups go at 6 weeks and others not until 8 or more?

In some states it is illegal to take a pup away from it’s dam before 8 weeks of age. They start eating gruel at 3 weeks, but most will still be sneaking a sip off mom for a good month after that. So for proper nutrition, and to maintain their immunities (which they get from Moms milk) until they can be vaccinated, allowing them to nurse is essential. Shots shouldn’t be given before 6 weeks and it takes time for the pups body to respond to the shots after they’re given.
Another major factor in my opinion is that during that extra 2 weeks, the pups are learning from mom how to be decent well adjusted dogs. If you’ve ever watched a pup attack it’s mother too roughly and seen her reaction you know that mother dogs will discipline their pups. They teach them when to back off, that they are NOT in charge and they have limits and expectations. After the pups leave mom they already understand when you say no to them that they must accept that. They have a good foundation to deal with the natural pecking order between themselves and other animals, and are more willing to be submissive when needed rather than become little bullies.
My pups don’t leave mom before 8 weeks and she decides when it’s time to wean them. The result is happier, healthier dogs. Usually toy breeds are older when leaving Mom because due to their size they’re more fragile and need that extra time.