Because, see, as written, Gon is a filthy, selfish little traitor. We've spent the whole story seeing the bond between him and Killua grow, seeing what it's based on, seeing how iron-clad it is, and then we see Gon turn his back on Killua, deny him, lash out at him and ignore how selflessly Killua is supporting him, all for the sake of a man who, as written, means next to nothing. An old buddy. Someone who gave him hunting tips. That's all we're shown. And for that, Gon drags Killua, Killua who has repudiated his whole previous life for Gon's sake, through hell and finally tries to kill himself in front of Killua.
And it would have been so easy to fix! Just a sentence here and there, creating a persistent thread, about how Kite is Gon's only living connection to his father, stood in his father's place, taught him things like morals and ethics, is the one who made Gon who he is (the person who could pull Killua out of the darkness like he did). Just some subtle reinforcement of that, at key points, and this whole fiasco could have been fixed!
But no. We don't get shown any of that. All we get shown is Gon betraying the one relationship we have actually seen developed in depth. Total. Writing. Fail.
Fuck's sake, it's enough to make a person start shipping Killua with fucking Hisoka.