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Because talking about other groups would be an admission that Israel's war is against the people of Palestine and wouldn't fit the narrative Israel and its western handlers are pushing of the war only being against Hamas.
It never made sense, but it's making even less sense now that the campaign has expanded to the West Bank, where Hamas isn't as dominant a force as it is in Gaza.
Plus, when it's just a narrative of "Israel vs Hamas", it's easier too keep up the idea low-information viewers have that this is a conflict between two equally proportioned groups
Keeps the narrative simple and erases the political and existential nature of the struggle by collapsing it down to Evil Mooslems vs The West 
If westerns knew there were like twenty groups with different beliefs and politics united in to a literal Rebel Alliance they might ask the wrong questions.
Alternately; The News Media are just dumb as hell propagandists huffing their own farts who don't really give a shit.
Both of these things may be true.
The MSM mentions PIJ pretty often as well. The point is to make it seem a like a religious conflict between (scary) Muslims and Jewish people, as opposed to a national liberation conflict between settlers and natives.