Itsa question i have raised before but there have been a lot of 747F crashes compared to the vastly more numerous pax versions.
There are aspects of pure cargo operations that can make it potentially more hazardous than hauling SLF. For example, as someone else noted, the three most recent 747F crashes (prior to this) were due to cargo fires or shifting cargo - both of which are a significantly greater danger during pure cargo operations. Freighters tend to operate much closer to MTOW/MLW and with more cycles (trading cargo for range with planned refueling stops).
All that being said, the 747F hull loss rate is still pretty respectable. Also, it's no longer the case that passenger 747s are more numerous - the majority of 747s currently in operation are either pure freighters or freighter conversions.