Why would people prefer choice band/specs, choice scarf, or expert belt over the other two?

Question

They're all used on Pokemon with all attacking moves. Why would one be better than the others for certain movesets?

Comments

"Why would one be better than the others for certain movesets?"
lol..what?
if you have a Pokémon that has ONLY attacking moves, with no status, then expert belt would definitely be good to use. or if you had only special/physical attacks, then use band/specs. or if you're wanting to be fast, use choice scarf.

no offense, but seriously, this should be obvious. this is the same thing as asking "Why would you choose Regice/rock/steel/gigas over the others?"
you pretty much asked "why isn't plastic glass?" when you said "Why would one be better than the others for certain movesets?"
it all depends on what pokemon you're using and what role you want them to have on your team, which gives you the idea of the moveset, which gives you the idea of the ability and/or item. This could also be seen as "Treecko, Torchic, or Mudkip?" for an example.

Answer 1

They're all used on Pokemon with all attacking moves. Why would one be better than the others for certain movesets?

This should be obvious, but okey dokey.

Let's start with Choice Band/Specs vs Choice scarf. One grants power, the other Speed.
It depends on which is better, power or Speed? Well it depends on the Pokemon, not just the moveset. Why? Because we use choice items to make Pokemon a threat. Choice Band/Specs grant the holder higher offensive power, allowing it to get a OHKO or 2HKO on Pokemon it previously couldn't, and putting a dent in Pokemon that switch in, sometimes allowing teammates to finish them of much more easily.
Choice Scarf grants the holder Speed, which is a very important stat in competitive battling. outspeeding opponents is important, and to sweepers, usually crucial. Even if a Pokemon is fast, there are often Pokemon faster than it, with the ability to knock it out before it can attack. These Pokemon that can't take a hit but can safely knock out a Pokemon are called checks to that Pokemon. When holding a Choice Scarf, though, certain Pokemon can check Pokemon that they previously couldn't. A good example is Terrakion, which often holds a Choice Scarf to outspeed Pokemon that would otherwise check it, including Mega Loppuny, Mega Metagross and Gengar. it also allows it to outrun the Dragon Dancers of OU at +1 Speed.
This way, Speed can make certain Pokemon more of a threat than pure power can.
Of course, there are Pokemon that benefit more from power than Speed, such as wallbreakers.

Now we'll move on to Expert Belt vs Choice Band/Specs.
Choice items give more power with a 50% boost, but Expert Belt can be preferred for Pokemon that have an array of coverage moves but like to stay flexible. A good example is was Greninja. Although Life Orb was usually preferred, Expert Belt was still a good item for Greninja, because of it's access to useful coverage moves. Even with its slightly lackluster Special Attack, with its coverage moves its attacks were often super-effective. Giving Greninja a Choice item however would have restricted its ability to constantly switch between coverage moves (and types).
Sadly, Greninja is now banned to Uber, where it serves as a suicide Spiker. Unless you're playing VGC.

TL;DR: It depends on the Pokemon wether power or Speed make it more of a threat, and because choice items can hinder the ability to cover a number of Pokemon at once.

Comments

yes, I have noticed that choice scarf landorus carrying stone edge are starting to annoy my thundurus.

Answer 2

Power.

That's it. Unlike the actual game, the meta-game is all about raw power (or, even the ability to powerful hits, but not relevant here).

Now, the reason Pokemon have 4 varied moves is to improve their coverage, and cover their weaknesses.

In the fast-paced meta-game (competitive battling), it is generally wiser to switch into a Pokemon that can resist an attack, and then retaliate super effectively.

Thus, switching is fairly common and staying in is not necessarily helpful, as it sometimes is in the actual games for EXP points and so on.

So, to be honest, the being locked-onto-only-one-move is not that big a problem; Even if it is, the benefits of the 50% boost are greater for the most part.

So, people tend to choose this power over being more diverse, simply because the Pokemon that switches need not only resist their moves, but also KO them instead. Nowadays, the latter situation is more common, due to the increase in the offensive spectrum of battling, as opposed to defending.

That is to say, in simple terms, people find it easier to attack than to defend.

Due to this reason, and that a 20% boost is no longer enough to overcome the opponents (highly EV trained and good item holding) Pokemon.