because they more or less used SD45's and to some extent SD40/-2's for their expedited service, not 4 axle units
at the time they just bought their 4 axle units for locals
by the time they changed this philosophy in the early 80's, they opted for GP50's rather than GP40-2's. I bet they regret this purchase...
later they (NS) bought a good number of GP60's and got some GP59's too
and of course they (NS) got some GP40's from the N&W and ended up with some GP40-2's from conrail