Why do cargo airplanes fly fewer cycles then airliners?
It's due to the overnight parcel distribution model which limits the available operating time.  A parcel picked up by 5pm needs time to get a terminal, and then has to get to the destination distribution terminal in time to be distributed and dropped off by mid afternoon the next day.

This means the heavies in the freight business do their flying overnight running between their distribution hubs (being a "Freight Dog" pilot is considered a night shift sort of career with fewer hours flown per year than pax flying, maybe 300-500 hours vs 700-1000 flying pax) so that freight can arrive at distribution points in the morning where it goes out to sub terminals on smaller aircraft during the day, or sent out by truck.  

For example, Fedex runs aircraft in and out of Atlanta only between 10pm and 6am.  For many years I worked at an airport with a Fedex Terminal and you would wonder how anything got done because all you ever saw during the day was parked MD-11s lined up in front of the terminal, apparently just sitting there, with no freight movements to speak of after about 7 am, except for smaller a/c flying sub-distribution runs through the morning.  Well they are actually just waiting for their outgoing freight to arrive and if you happened by at midnight, it was pretty busy and there was a fair number of a/c coming and going.

So a freighter has maybe an 8 hour operating day, whereas a passenger a/c gets flogged like a mule and starts flying at 6 am and runs until 9 pm.  Plus the loading and unloading process for pallets is somewhat slower than with self-propelled human cattle getting on and off for you.  So you can't really do more than a couple of legs per day.

To provide context, this UPS fact sheet shows a fleet of 255 UPS jets doing 318 daily trips on average 

It says "In-Out" trips for that 318 number, so lets assume that's actually 2 legs, 1 in and 1 out, for 636 actual flight cycles.  That's only 2.4 cycles per day overall, almost as easy a life for the airplane as corporate.