It's not that many aircraft in fact. They are now very cheap to buy (although fuel costs per kg. hauled must now be high), but basically the market is obviously smaller in Canada than in the US, where the major package/freight companies have moved on to large fleets of widebodies, and in some cases 757s.
In fact if you want something smaller than a 757 there is not a lot of choice. The 737 doesn't seem to hack it for freight work, there have been relatively few conversions, and none from the MD-80 or A320, which all seem to go for scrap when the passenger airlines are finished with them. I guess the extra engine comes in handy for the higher gross weights cargo operators tend to go for.
Note these are freight aircraft. Canadian passenger airlines came late to the 727, Air Canada preferring the DC-9 only for some years (another example of wanting the smaller aircraft) and CP/Canadian followed a similar course.