So what is the real reason ?
Low frequencies with moderate sized coils means low/moderate inductive reactance and, when parallel tuned with a capacitor, large circulating currents can be made to flow (amps) without the expense of that current being sourced from the battery.
A much higher frequency means a much smaller current flow for a given applied voltage.
High current means high magnetic field; low current means low magnetic field.
More magnetic field means deeper penetration.
However, higher frequency means higher rate of change of flux and this can certainly counter the problems mentioned earlier.
However, the main reason why detectorists prefer low frequencies is that the unwanted effects of a wet-ground signal are much reduced. These are electric field effects between transmit and receive coil and are unrelated to the magnetic fields produced but, are significantly more problematic at higher frequencies.
GPR generates a pulse EM wave and, EM waves reduce in amplitude with \$\text{distance}^2\$. Magnetic fields (that don't form part of an EM wave) reduce in amplitude with \$\text{distance}^3\$ (beyond a certain point) and so are less able to penetrate deeply compared to a pulsed RF.
GPR emits a thin TX pulse that turns-off before the RF reflection returns. A regular metal detector has to both transmit a signal and look for the tiny return signal coming back and, be able to detect it in the presence of the continuous transmit signal (much trickier in terms of signal to noise) but, it gives you the opportunity of being able to discriminate between ferrous and non-ferrous signals (important for detectorists).
GPR will detect rocks as well as metal objects. GPR will also detect strata layer changes. Do you really want those "features" when looking for Saxon buried treasure in Staffordshire fields or would you prefer to discriminate against these unwanted things in favour of metals?
