That's because regular milk lasts a week or less, while organic lasts about a month, says Scientific American. The reason behind organic milk lasting as long as it does is because it's ultra-pasteurized. That basically means the milk goes through pipes that are heated at a much higher temperature than regular pasteurization. The milk is also heated and cooled more quickly. The downside to that is the taste and texture may be affected through ultra-pasteurization. Apparently, since some of the sugars in the milk are burned or caramelized because of the higher temperature, ultra-pasteurized milk is sweeter. "[Ultra-pasteurization] also destroys some of the milk's vitamin content — not a significant amount — and affects some proteins, making it unusable for cheese," says Scientific American. On the upside, "milk that undergoes [ultra-pasteurization] doesn't need to be refrigerated and can sit on the shelf for up to six months."
If you're wondering why organic milk has to go through more extreme heating process, it's because organic products have to stay fresh longer since they have to travel a further distance. Organic products aren't produced all over the country, so they tend to take more travel time.
Technically, you don't have to get organic milk, just one that's ultra-pasteurized. But regular milk that's been ultra-pasteurized is harder to find, says RoamingAmber. However, if you're not fussy about your products being organic, you will be able to save even more by buying nonorganic ultra-pasteurized milk.