Why do LF RFID tags use near field and why is reading distance longer with UHF RFID?
It's about catching the radiating energy - transverse electromagnetic wave, which is very small at long distance. Therefore a long range RFID has to have its own power supply, battery like. The received signal is a weak radio signal.

Meanwhile the LF tag works in near field, where magnetic coupling is possible, so it can be powered from the field without using its own supply. Further the transmit antenna could be also sensing the receiver within near field, which for far field isn't possible at all. When the EM wave is forming it is still binded to antenna in the conductive region- near field, but when it is 100% formed it becomes a transverse EM wave and it leaves for ever, the transmitter can't retrieve it back anymore.