By now there have been only a few conversions of TriStars into freighters, while almost every DC-10 has been converted. The only 'bigger' Freight Line using a L1011F is Fine Air. Correct me if I'm wrong, but I think the safety record of L1011s is better than the ones of the DC10s. The only big L1011 crash I remember is DL191 (shearwinds). I think the L1011 has always been more advanced than the DC-10. There are for sure more DC-10's at the market, but in the recent a lot of L1011 have been retiered from pax-ops (Cathay, Delta, Saudia, Royal Joradanian.....), most of them became victims of the blowtorch instead of being converted into a freighter.
So why are there almost only DC-10F's???