Here is one way of looking at this. To start, the reason Life Orb is chosen over items like Black Glasses is for some combination of these reasons:
- Life Orb provides a stronger boost than Black Glasses, etc. This is needed on some Pokemon because it secures important OHKO/2HKO on relevant threats.
- Most offensive Pokemon use attacks from more than one type (lest they be walled easily), and require a damage boost on those moves to make them hit damage benchmarks. You could boost them using an item like Black Glasses, but then your spammable STAB move is less powerful. Life Orb lets you do everything.
- Offensive Pokemon tend not to mind the chip damage from Life Orb too much. It's obviously not ideal, but given that offensive Pokemon are fast and usually switch in off advantages created by teammates (e.g. when a teammate faints, when you make a good prediction, when you use pivot moves etc.), their ability to tank a hit is not as important. (Ideally, they'd make the opponent switch or force a KO, and neither of those scenarios involves taking damage.)
So, the reason Black Glasses is chosen over Life Orb in rare cases is because the above reasons are less applicable to the Pokemon in question. In the case of Bisharp:
- You will probably find by running damage calcs that Bisharp's attacks, especially at +2, do enough damage with Black Glasses to KO most of UU. Life Orb's boost would still be nice, but it isn't necessary.
- Bisharp is almost always going to be using its Dark STABs. Sucker Punch lets Bisharp sweep Pokemon that are faster than it, and Knock Off lets it break defensive Pokemon that can play around Sucker Punch. That's a lot of utility coming from one type of move. I looked through the early December UU stats, and I could only name a couple Pokemon I'd use Iron Head on instead of using a Dark STAB or simply switching out. Most offensive Pokemon aren't like this.
- Despite being a set-up sweeper, Bisharp kinda likes having extra HP. Its high Defence and part Steel-typing means that occasionally, it's a sensible idea to hard-switch into it. I'm not super familiar with UU, but after a flick through the usage stats, I can see how you might want Bisharp to duel Pokemon like Jirachi and non-Superpower Scizor. Bisharp also relies on Knock Off or Iron Head to hit some Pokemon, which means it could be outsped (or the target could simply survive its attack). For example, extra HP would buffer against things like Tentacruel's Scald and Azumarill's Aqua Jet, or Pokemon like Hippowdon and Celesteela that survive Bisharp's STAB and hit back with attacks that do ~80% damage.
- You might argue that boosting Iron Head is a good idea for a situation where your Dark STAB won't enable a sweep, but I'd argue that you shouldn't teambuild around scenarios like that. If you can't get Bisharp using its Dark STABs then you shouldn't be trying to sweep with it to begin with. You should focus on enabling Bisharp's most effective tools.
None of this is to say that you have to use Black Glasses, either. If your team has ways to hit the targets that pressure Bisharp into using a more conservative item, then you might end up running LO anyway.
E: Yes, there are other situations where you might choose a 20% boost item instead of LO. One example is Bulk Up Urshifu, which is designed to stay in play for a long time (because it's attempting a sweep) and will potentially run into similar obstacles as Bisharp (e.g. bulky Pokemon that want to chip it down). I can't think of any examples where a 20% boost item is the unanimous best option for a particular set, but there might be one in a format I'm not familiar with (esp. doubles/VGC).