Why do lower frequency RFID readers have a smaller range for reading tags?
A UHF emission is likely to be received as a true radiated electromagnetic wave. Close up to the transmitting antenna it's called "near field" and the individual E and H fields aren't really combined/lined-up until beyond about one wavelength. A wavelength of 300 MHz is 1 metre so if you are metres away you will be receiving a true EM wave.

True EM waves reduce in received power as distance squares. As individual fields, E and H reduce linearly with distance - remember this....

For a low(er) frequency RFID reader such as one operating at 14 MHz, one wavelength is 21 metres hence you are never going to be receiving a fully-fledged EM wave. Instead you will receive just the magnetic field of the transmission (because it uses a coil and there is very little E-field content transmitted).

Beyond a distance that is approximately the diameter of the coil\$^1\$, the magnetic field reduces with distance cubed.

Hence scenario 1 (UHF) the magnetic field reduces linearly with distance and in scenario 2 (14 MHz e.g.) the magnetic field starts to reduce as distance cubed.



\$^1\$ - here's the background for the magnetic field reducing with distance cubed: -



If z gets much bigger than R, the "R" within the parenthesis in the denominator can be ignored leaving just \$z^3\$