Although IMR has LOTS of downsides compared to LiFePO4, the reason that it is preferred around here is that it's a drop-in replacement for ICR. Flashlights are generally designed around 3.7V chemistry rather than 3.2V chemistry. Because of this, IMR is preferred to IFR, despite all the advantages of IFR.
Of course, what we REALLY need is a better 3.7V battery, with all the advantages of LiFePO4 (like safety, constant voltage discharge, long life, etc) and good capacity. But whether improvements in current 3.7V chemistries can bring this about or whether new chemistries with these advantages can be found is another matter.