Why does the resistance of an 8x8 LED Matrix have to be on the cathode?
In a LED matrix you're either scanning rows or columns. If you're scanning columns the resistors have to be on the rows, and vice versa.

Scanning columns as is done here means that one column at a time becomes active. Then for each LED in that column you have a corresponding resistor on the rows connection.

Regarding Michael's comment: if you would put the resistor on the scanning side (here the columns) then you'd have 1 resistor for a complete column, the one you're driving at that moment. Then the brightness of the LEDs will depend on the number of LEDs you have on. If you have 1 LED on it will shine 6 times brighter than when you have 6 LEDs on.