There is an important hardware difference which limits the speed on I2C.
SPI: all lines are driven by the transmitter both high and low. This minimizes the time required for the wire to change states.
I2C: all lines are open-collector which means that the transmitter only drives the line low. When the transmitter releases the line, a resistor connected to Vcc (supply voltage) pulls the light high. However, due to capacitance of the wire and the components, the wire goes to high voltage relatively slowly. Because of this, the clock speed must be reduced to allow time for the lines to "drift" high.
