Women are often looked at as nurturers rather than someone who would kill. It's also believed that they are anatomically inferior to their male counterparts, so they would be ill equipped to carry a fallen comrade, their packs, and any additional equipment.
These arguments will become obsolete in the near future with an emphasis on remotely operated drones, and tactical gear that acts as a strength enhancing exoskeleton. :) We spend money on military like no one's business in the USA, seriously, we blow our cap on our budget because it's just flat out ignored.