The other parties aren't standing in this particular byelection because they (correctly) realise that it's just a media stunt by Farage, who is attempting to paint himself as a victim of establishment harassment (while being the epitome of 'the establishment' himself). By not standing, and forcing Farage to run against joke candidates, they've made a fool of him (which generally harms populists more than anything else you can throw at them).
The other parties have all said that they will run candidates against Farage in the almost inevitable second byelection that will by triggered later when parliament finishes its investigation into his corruption and punishes him for it.
The other parties have all said that they will run candidates against Farage in the almost inevitable second byelection that will by triggered later when parliament finishes its investigation into his corruption and punishes him for it.