Because Gun Game should be a bare bones kind of mode. It's a true test of skill when it comes to melee and shooting. You start adding items like meds, speed vials, and mines then it just becomes your run of the mill Deathmatch with randomized guns.
Why would you want meds in this game mode? All the extra items would just acr as disruptors and take focus away from outskilling your opponent with the use of melee and guns.
Why am I focused on sticky nades? Because I look at sticky nades as a sort of skill shot item. Grenades are also somewhat of a skill shot item, but not as precise as a sticky. I wouldn't mind seeing both frag and sticky nades added into the rotation. But mines, flash, molotov, smoke, tnt, speed vials, and meds I personally feel like they would take away that bare bones feeling when you join a gun game room. You say that they're used in every other room already, which is ttue. But it's like when you host a Vanilla room. You're hosting that vanilla room to get away from all the high stat players. Same goes for gun game in my eyes. You make a gun game room to escape all that extra stuff to just focus on shooting and meleeing.
You say that snipers and rockets barely see use in normal rooms. Then you say that meds/items are used in every other room. So why would you want to add the things we see in every other room, to the gun game mode where adding those things would dilute the item rotation and make those less used items see even less use in gun game?