The reason is totally tactical. General infantry historically gets cut to pieces in terrain similar to Afghanistan. Special Ops like Rangers and Commandos are more mobile, flexiable and easier to keep supplied and prootected.

If I recall correctly, the US government was going fo a mass deployment in the region, but the military suggested a smaller, more specialised deployment would be more effective based on the recent Soviet experience

Both the British and Australian governments arrived at the same conclusion. So rather than being an odd decision, in fact the US made a very intelligent and measured choice

Hope that helps