Why is that the gate current cannot be negative for SCR?
I don't know what you mean by "can't be negative". If the SCR is off you can reverse bias the gate, and a small amount of negative leakage current will tend to flow (depending on what anode bias voltage you have etc.). 

For example, the 2N5064 is rated to handle as much as -5V on the gate. 



If the SCR is on, much more current will tend to flow (depending on the anode current), but this is not a well defined region of operation for most SCRs. Some will commutate (turn off) if you suck enough current out of the gate, but it's not guaranteed and I suspect it could damage the device under some circumstances.