It's simply because CN has higher spending priorities than repainting.
As such, although the railroad may have a single "corporate standard" paint scheme at a given time, it can take *a while* to accomplish unity. CN is not alone, viz CP which not only has a couple of CP proper schemes running around, but units still with MILW black beard and SOO paint to boot.
This phenomenon is not unique to RR's; many airlines will have planes in >1 scheme flying around for several years as they phase in the new image.
The many different paint schemes allow the railfan to know when the CN Locomotive was last serviced and modified. That helps a lot.
Andrew
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