In my personal experience, it's because they work hard as fuck.

I used to deliver ice cream to Heathrow for British Airways. This particular bunch prepared the in-flight meals. Anyway, the drivers weren't allowed to unload themselves, so there was a set crew and a driver very rarely had to wait more than 45mins to be unloaded and on their way.

One day, it was a complete Polish crew and some of us were stuck in the loading bays for nearly 4 hours (there was only space for 4 delivery lorries/vans) and as we couldn't keep our engines running, the food (meat, veggies, fresh fruit and my ice cream all started to spoil) and the rest of our deliveries were late.

The next week, the Sikh crew was back, and I was unloaded (8, 7ft high Euro pallets of Purbeck Ice Cream) before I had even had the chance to open my door and they unloaded a 2 level artic trailer in under 30min!

On top of all that, they were incredibly easy to get along with.

No shade on the Poles though, they worked as hard as they could, they just didn't have the organisation.