Regarding personal plate carriers, if an item has a NSN, it means it was or is issued, correct? Why wouldn't it be able to be used?

Some items may be appropriate for some situations, environments, or units that aren't appropriate for others. A plate carrier with an NSN may not provide the protection needed (as deemed by a superior in your CoC), though other units may find it necessary to sacrifice some protection in order to lighten the load.

Some outlandish examples to illustrate the point -

I decide that I want to be as protected as possible as a light infantryman, so I purchase a EOD tech suit and wear it on patrol. It has an NSN, but it isn't the right level of protection for the situation.

I work at the CDC with highly contagious viruses, but my PAPR suit is cumbersome. I buy an M40 pro mask and wear it instead. It has an NSN but isn't the appropriate tool for that environment.

Obviously the difference between an IOTV and a plate carrier isn't as big a gap as those examples, but the point is, someone above you and me decides what is an appropriately level of protection for the situation, environment, and unit we are in.

Could be old, outdated or found to be flawed but mistakenly left in inventory. Could be for use in specific conditions. Could be prioritized to different units.

Because...Army.

I have worn issue, non issue, NSN non issue, ect. It all comes down to your unit.

Want a reason why? Because someone in a command position somewhere in your CoC decided so. That does not mean it was a logical, or illogical decision; they honestly may just not care that much and it's easier to say you wear what you are given.

As a young Soldier, I promise you....you don't want to be that guy going against what everyone else (especially your SGT and above) are doing/advising. Logic dictates, a fighter is more effective if they are comfortable in the gear they use, let alone the benefits of a Crye AVS vs some issued bs. But, the Army (it's typical "leaders") don't care about that. Just be happy you don't have 1 of those unit SOPs that dictate where each pouch on your kit will be located, and force you to wear 1 standard loadout across the formation....

To clarify, NSN stands for NATO Stock Number. That means the item has been purchased through formal procurement paths by at least one NATO country. It does not mean it has been issued, that it is in normal use, or authorised for use by US forces. There are Sikh turbans with an NSN. That does not mean you can wear it with your uniform.

The Crye AVS has an NSN because it's standard issue for USASOC. Conventional force has different standard issue stuff. The Army said many years ago (like 2007ish) that you HAVE to use ISSUED body armor or you forfeit your SGLI. ARSOF gets the AVS for a reason, the mission requires it and Humans are more important than Hardware. The conventional gets not-the-AVS because they get what is the least expensive thing that meets the minimum standard - because they have to outfit 300,000 Soldiers with it. Think 40 person Infantry platoon over 12 person ODA or 4 person CAT.
The IOTV blows, trust me I know. We all wanted Dragon Skin when it debuted but it wasn't worth forfeiting SGLI for a little comfort.
If you want an AVS, head on down to bldg 1042 on O'Connell Blvd and ask for Larry, Cesar, or Will and they'll get you prepped for one of the 3 Selections.

Believe me when I say you do not want to look different than those around you. A sniper scanning your column looking for a high value target would be attracted to someone who doesnt match.
Wear what you're issued.

I'm going to be the turd in the punch bowl here. Ok, you get deployed to the CZ and bring your Crye stuff, and several of your buddies, based on your example, decide to bring their own personal favorite armor(s), all different brands, configuration requirements, etc. Your unit comes under fire and several of the armor systems get their plates broken. Where are you going to get replacements? Is Crye & every other non-standard system going to be authorized to ship their stuff to you? Probably not. So you and several of your buds are now without effective armor and possibly wounded. Having deployed to Kandahar as a USARC G-4/8 liaison, I can tell you that the CIF there was running minimal inventory and would have to contact Ft Benning's CIF to get shipments to them. Now you & your buds have to go out with insufficient armor. Do you think your CO & 1SG are at all happy with that solution? No, they're not, they're even having to think about how to explain to your parents how this all happened.

Several questions here: Items with an NSN (National Stock Number) are generally furnished through military channels but are also available through Clothing Sales Stores at PX/BXs and off-post Army/Navy stores (new & used, etc.). The question of usability is different, as something may not be allowed due to command directive or possible damage to the item. Can you provide more detail on the situation?