RTL NOR gates work because, in an RTL circuit, BJTs are basically current controlled. MOSFETs are voltage controlled.
If you sum 3 identical resistors into the base of an NPN, current through any one of them can turn on the NPN.
If you sum 3 identical resistors into the gate of an NMOS, a high on only one input will not turn on the NMOS unless you use a high supply voltage, because of the typical threshold voltage of MOSFETs and the voltage divider created by the summing resistors.
See the attachment. Note that with one input switching, the NMOS circuit does not work with vcc=5V, while the NPN circuit works with either vcc.
Also note that I used devices with typical specs, not worst case. The NMOS circuit might not even work with vcc=10V if worst case Vgs(th) were used.