If you're not familiar the maillard reaction is basically the browning of food like the crust on a well seared steak or the crispy outside of a cookie. If gives you a deeper, more complex flavor. Some people call it caramelization, but that's not really correct because it's chemical reaction involving amino acids, not just sugars.

When you use a slow cooker, your temperature never exceeds boiling, which is not hot enough to induce maillard reaction. You don't have that restriction in a dutch oven. Pressure cookers work by raising the boiling point due to the pressure. So you're still limited by boiling, but the boiling point is high enough that you still get some maillard reaction.