Why does lactose-free milk last longer than regular milk?

While the process of converting cow’s milk into lactose-free milk doesn’t extend its shelf-life, dairy producers commonly utilize special pasteurization methods that keep lactose-free milk fresh longer, mindful that as a specialty product it may not sell as quickly as regular milk.      
 
Lactose-free milk is pasteurized at a higher temperature than regular milk. The process, known as ultra-pasteurization, is designed to remove the bacteria content entirely, giving lactose-free milk a refrigerated shelf-life of 60-90 days, compared with regular pasteurized milk, which retains some bacteria. It has a shelf life of 1-3 weeks.
 
Source(s):
The FDA Center for Food Safety A-Z Reference Guide
Emil Nashed - VP of research and development/quality assurance, Farmland Dairies
International Dairy Foods Association, “Pasteurization: Definition and Methods”
National Dairy Council Factsheet: “Newer Knowledge of Dairy Foods--Protecting the Quality of Milk and Other Dairy Foods”