About the freq limit: consider that the resonant freq depends on the
applied magnetic field, and you can't perfectly shield the osc from ext
fields: power line frequency things, Earth's field, etc. Since f is
linearly proportional to the field strength, things are percentage-wise
more immune to external fields if the field strength is high, and thus
the resonant frequency is high. Also, other designs are relatively
more practical at lower frequencies; it's no great trick to make a
lumped LC oscillator at a couple hundred MHz (though electronic tuning
over an octave range becomes a bit of a chore because of all the stray
capacitance). So it's a matter of the frequency range over which they
are relatively more practical and economical than the alternatives.
And of course, there are many factors to the "practical" part of that.

Cheers,
Tom