Why does the NASA NEXT engine use xenon? Why not use argon or krypton? Does it have to do with the thrust to power ratio or the specific impulse or something else?
This video explains it.
In short, it all boils down to golden middle between energy requirements, thrust and specific impulse. Ionizing lighter noble gases requires much higher potential - much more energy input, larger solar batteries, higher cooling requirements, circuitry and structure toughened against higher voltages and energies. While specific impulse would improve, it's already very high, meanwhile thrust is already low, and it would drop even further.
There are secondary advantages - containing xenon is easier, the large particles not as prone to filter through micropores of the tank and more mass storable in the same volume at the same pressure (and while propellant volume itself isn't that much of a concern, the tank for same pressure and higher volume is necessarily bigger - and heavier.) Xenon is not radioactive, like Radon, and being a noble gas is very non-reactive, safe to the structure. But its ionization potential is the primary reason - still reasonably low, while providing excellent performance.
Surely if currently achievable specific impulse appears to be insufficient for our goals, we'll move to lighter noble gases, but for now all destinations within the solar system are reachable on xenon engines, and the advantages of argon or krypton just don't justify the trade-offs.