Raspberry Pi not Compatible with Mosfet?
why does this (circuit) work with the 5v pin and not the 3.3v gpio pins (of the Raspberry Pi)? 


You should certainly expect that the RFP30N06LE should work with a 3.3V GPIO signal from the Raspberry Pi. 

The datasheet shows the typical characteristics in Figure 7"



NOTE: The graph shows that as you require less current the gate voltage becomes less important. From the graph you can see that it does not matter whether you have V(GS)=3V or V(GS)=10V if the current flow is limited by the load to &lt;1A. The voltage V(DS) is a simple voltage divider formed by R(l) and R(DS). The device is operating in the Ohmic region I showed in the Red circle. See Figure 16 and 17 in the datasheet.
This Appnote might help some.  

You will not get the minimum RDS(on), but you would expect with a 12V VDD and less than 1A load current to see more about 0.2-0.3V on voltage. 

However, the device has been obsolete for many years and if your source of parts is Ebay, the chances are it is NOT an RFP30N06 at all, but some substitute part.
The official replacement device for the RFP30N06LE was the HUF76423P3. This device (which is available) has a VGS(th) of 3V max so is definitely likely not to work with a 3.3V gate drive. 

I would suggest that you should connect your device gate direct to the R'Pi 3.3V supply (Pin1), and if it does not turn on your fan then the VGS(th) is high enough to prevent operation.

You can use a device with a higher VGS(th) by changing the circuit slightly. 







simulate this circuit &ndash; Schematic created using CircuitLab

Here the off voltage is about 0.7v (less than minimum VGS(th) in the datasheet) but the on voltage is about 4V, which should be enough to activate your device if it's VGS(th) is anywhere near 3V.