They come up with new radios all the time. RF Chip makers also come up with new chips with lower consumption all the time.
I was not aware of this RFM66. I see the registers are different, the major improvement would be the RX current, from ~16mA to 9mA (with no LNA boost).
The pinout matches that of RFM69HCW/LoRa radios so from a PCB standpoint you don't need a new revision to change from RFM69HCW/LoRa to RFM66.
To answer your question why - I guess it's because it's probably quite new and there are no libraries for it. Here's a chance to write one
UPDATE: also the RFM66 has no hardware encryption apparently, so the RFM69 would be superior in this area and you'd have to do this is the MCU if you wanted encryption, it would be much slower for 8bit MCUs and you'd spend lots more clock cycles and power doing that for the 66W, whereas the 69W this can happen in hardware very quickly.