The reason people don't use it is because a lot of people don't have it unlocked.
I actually really like it in gun game but honestly the grind to level 11 with the Ballista for a non-sniper is pretty long and for me at least, boring.
I hate sniping in most CoD games due to map size. They just aren't big enough unlike say Battlefield. Sure most maps, even Hijacked, have decent line of sights but due to the terrible spawns and just fast killing weapons in general, sniping puts you at an immediate disadvantage. This is without mentioning the terrible hit detection/lag nonsense that renders sniping at times mute.
I perma unlocked the Ballista this prestige and have been grinding to get the irons but it is just a chore for me. I did finally get the ACOG and it is going a little faster. Also helps that the OSOK camo unlocks are pretty easy so it makes it go a little faster. I think I made it up to extended mags over the course of 2 days (I used it for like 10 kills in a prior prestige).
Looking forward to finally unlocking them maybe this weekend but I am definitely not prestiging the gun or even trying for gold. I don't snipe enough and I don't care enough about a camo to even try to use this thing with a scope again.
"Cover your receptors Perceptor." Driver picks the music. Shotgun shuts his cakehole.
The sights themselves are huge and provide terrible sight, the swat is too severe and you cannot hold breath to steady. All of that eliminates its range (IE the whole ****ing point of a sniper) and it scopes in and re-chambers at the same speed so it provides to CQC advantage.
Ballista Iron Sights is very fun to use, but only in a "haha, I just defeated you using an inferior weapon" kind of way.
Not unlike running knives only, pistols only, etc
Like most have said here, it's practically a worse version of the KSG...the KSG can aim down sights quicker, OSK anywhere to the body (within range), better hip fire accuracy, huge magazine...the Ballista's only advantage over the KSG is OSK from any range.
But even so, I love using Ballista Iron Sights and do just fine with it. Here is a clip from a game where I went 54-10 in kill confirmed. (naturally, ignore the theater mode crappiness where it looks like I was aiming behind the person)