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Remember that digital circuits try to approximate square waves, which have odd harmonics reaching many times the base frequency. A 10MHz clock, then, will have harmonics at 30MHz, 50MHz, 70MHz, 90MHz, 110MHz, etc. all the way up to infinity assuming a perfect square wave. As you can see, 90MHz is the only the fourth harmonic of 10MHz, so even if you're using such a slow clock, you still can suffer from high-RF effects.
The problem becomes progressively worse the higher your clock frequency too. A 14MHz CPU will have harmonics at 42MHz, 70MHz, 98MHz, 126MHz, 154MHz, and 182MHz. All of those frequencies lies in the VHF spectrum (in fact, 98MHz is right in the broadcast FM band!).
Note how progressively worse this is compared to a simple 10MHz clock. 4MHz makes a huge difference!
Minimizing lead length also reduces B-field coupling between circuits, as well as minimizing capacitance. B-field coupling doesn't require resonant antennas to detect or transmit (were that the case, our transformers would be huge). Additionally, reducing the inductance has the benefit of also reducing the opposition to alternating current, which means your higher frequency energy propegates through the wire with less distortion. This will make for cleaner square waves and steeper signal slews, a mandatory requirement for the 65816's clock at the very least, and probably also the 6502 too, particularly since these parts are engineered as if they'll be running at 14MHz, even if you never do so yourself.
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