Frequency response is plotted using a semi-log graph, which uses the same width for each octave. The octave between 500 Hz and 1 KHz is "worth the same" as the octave from 5 KHz to 10 KHz. This also more closely correlates to how we hear.
Is it mainly to aid in crossover design? I think it odd that 10-20hz get's a big gap while 900hz-1khz get's a small gap.
+1 to the above. In your example, the 10 - 20 Hz difference is an octave. If you could find a piano that went down to 10 Hz, you'd play all twelve notes of the western musical scale to get to 20 Hz, one octave up. On the other hand, from 900 to 1000 Hz is just about two steps of the scale (although no notes actually hit exactly at 900 and 1000 Hz with normally tuned instruments . . .
I'll add that the same applies for the volume scale vertically. Decibels are already logarithmic, so the horizontal lines are evenly spaced one above the other. But if you were plotting volts instead of dB, the vertical scale would also be log. That's called a log-log graph rather than a semi-log graph.