The answer is because you are a noob and whining about getting killed instead of admitting to yourself that you panicked or were otherwise unable to aim precisely and rapidly and were thus gotten the better of by your opponent.
Using certain weapons in certain situations is seen as a crutch and is looked down upon. Some examples include:
-trying to snipe at long range with a semi auto sniper. This is a crutch because the semi auto snipers are less accurate and do less damage, so most of the time to make kills at long range you have to spray and pray. Since most of your kills are thus the result of luck, you are looked down upon as a noob.
-using full-auto shotguns. These give you a high likelihood of making a kill at close ranges, but almost guarantee you are killed yourself right afterwards. They are essentially kamikaze weapons. This is looked down upon because it does not help your team to make kamikaze kills, nor does it even give you a good k/d ratio. It is a crutch used by players that otherwise can't get many kills at all. This also applies to using the grenade launcher or RPG as a primary weapon, instead of just for getting people behind cover.
Yeah but, a lot of times people are using those simply to earn unlocks on a new weapon or, thanks to the encouragement of DICE, using them to complete Assignments.
To accomplish certain objectives, I was going around as an Engi with both the DAO-12 and RPG on a few CQ maps. Those RPGs were flying like crazy.
On a regular map, shotguns are indeed somewhat worthless and tend to result in kamikaze runs. The intent, if this appears to be the case, is to at least get 3 for one, or more.
Depending on the map, it's easy to go for longer stretches with a shotgun in CQ. On more open maps, you won't be alive long once you get close enough to hit a few.
I've also slowly been trying to raise my Recon score, I've neglected it since launch - haven't really enjoyed playing the sniper role and so I'm terrible at getting good sniping positions. Aiming I seem alright, but it's the rest of the part I'm not so good at playing yet.
So I've been giving myself the shotgun as Recon (CQ), got the T-UGS unlocked (I told you, I don't play that class!

), and got the one assignment done in like one or two rounds (get 20 assists with T-UGS) on CQ maps.
Normally I've just focused on playing and aiding the team, but I've been on an Assignment/Unlock spree myself. Trying to focus on the assignments that I've made decent headway on without focus, or otherwise ones that are simply enough to get in a night.