ooooh he's a new, hot, funny character with boss trident skills let's give him a proper backstory and plenty of screen time.
Mockingjay Part One:
uh-oh he's dealing with debilitating sadness and his Ptsd is showing let's throw him in the hospital and keep him there even though he's supposed to be a part of Katniss's film crew. We can replace him with Gale. Just pretend like we weren't filming the underwear scene because it has no importance beyond him being half-naked, right? And oh, look, he's talking about involuntary prostitution that's kind of crucial for fans to understand a victors lifestyle…but that's kinda dark stuff so let's just put it in the background and alternate between shots of Gale and Katniss.
Mockingjay Part Two:
welp, looks like he's getting married. Happy day. Let's focus on him and his wife for five seconds before turning the cameras back to Katniss because I mean she's the main character and we can't give side characters proper credit, now can we? Okay he's off to war now and we could be giving him some screen time because he has several heart-breaking reasons to be fighting against the capitol and he has an awesome backstory that we could dial in on but we have Katniss's face to focus on so let's just have him help Peeta in the background without being thanked once for it. Okay, it's his last scene, and looking back on what we as producers have done to him in the last two films we should probably give him a decent death scene. See? We care about our secondary characters! We gave him a good death scene!