There's a distinct lack of new clothes in the fallout world. There's leather, metal, hide, etc.  And pre-war clothes.  But I don't see very much in the way of genuinely new clothes -- they know what pre-war clothes look like, and seem to value suits, but aren't making new ones.  

Could cotton be extinct, and therefore something as simple as denim is irreplaceable?  The NCR at minimum is at a 19th century level of developement... except for clothes, apparently.  New Clothes, and I mean reasonably nice new clothes, have been accessible throughout human history.
It's been a long time since I saw Kimball in game, so I don't recall if he had a fresh, new suit, or just a fairly good pre-war one.  
Crocker is probably fairly well off by citizens of the wasteland, but was clearly wearing garbage.  
Now, for whatever reason, large numbers of fallout denizens just seem to not mind living in trash and ruin.  They don't clean up or build anything new, quite often.  That's just one of the cultural quirks of the game, like the 1950's with lasers stylization.  But Shady Sands was actually a nice looking place, from what little we could make out in the pixels of fallout 2.  It wasn't a trash heap with people wearing rags.  
So yeah, maybe cotton died off in the war.

It seems pretty likely at least in the core regions. To the best of my knowledge it is not directly stated whether or not the NCR grows cotton. Although cotton CAN be grown in California, not much actually is.

Spoiler:

That gives cotton less of a chance to be found in NCR simply due to it not being too widespread in the first place. Caravaning it in from other regions would likely not be a priority for most of regions history. Contemporary reasoning for not trading would be the NCR hostile surrounding: Legion, tribals, natural barriers.

There is also the possibility that cotton and other agricultural products in the fallout lore are GMOs. Given the fact that most GMOs are made to be infertile it is not particularly likely that many crops, or any commercial ones really, would be able to survive in the "wild" without human intervention. That is pure speculation though.

Taking raw cotton to cloth to clothing is quite a bit of work. A lot more work than refurbishing older clothes. Also, since there was a huge population drop and many vaults, warehouses and such that still hold a large amount of product, it is just more economical to use the older clothes than create new ones.

Keep in mind, even 200 + years ago they have processed foods... So there was very likely a massive surplus of product (food, equipment, etc) from prewar and now the primary occupation is scavenger, caravans and raiders...

Another thing to consider they have leather or leather options but still don't show much in the way of using Brahamn to create leather (where we are at in game) even though there are huge Brahamn barrons in the NCR

Likely what is being produced is being integrated into already existing clothing and other products.

It's entirely possible that cotton did go extinct and that, aside from refurbished clothes, it simply isn't used in clothing anymore. The wealth gap seems to be pretty massive in the wasteland, and I think that its far more likely that animals survived more commonly than plants, so lower class citizens might wear things made of leather or whatever modern equivalent of wool (if such a thing still exists, other cattle animals have been show to survive the war), while the rich wear silk (possible, since other insects seemed to have survived the war with a bit of mutation, why not silk worms?).

Admittedly, the entire production of any sort of clothing is a mystery in the wasteland, since we never see or hear anything about it. Its hard to even tell what some clothes are made out of. the NCR uniform in NV looks like it could be made partially from burlap, which is made with jute, which is not excessively grown in the US but is hardier than cotton. We just can't be certain.

If we assume cotton is extinct, that means that Mississippi would have been hit. Fallout: Mississippi anyone?

I would assume that places like the NCR would manufacture clothes and stuff. Since its said that the ranger uniforms are made in the NCR. It would be silly to say that there is a vibrant cotton trade across the country and its somehow being sent from the south to the NCR. If the NCR could get rails made and other industries going. Why not clothes.

I don't think California or the Mojave have any canon contact with Texas, Mississippi, or the southeast.  The ncr  didn't even know about the Legion until it invaded, apparently, and the legion territory is huge.  It's possible other areas of the country have different goods.  
Hell, i wish we'd seen this in the Eastern fallouts, such as a different currency, etc.  I have to keep in mind that anything that wasn't locally produced is probably not available anymore.  In modern times we're often unaware of how much is imported, even if just from another state.

Edit: The NCR armor according to the wiki apparently has khaki. Googling to make sure, apparently that's cotton. So Crocker using a suit that he appears to have found in the garbage is simply bizarre, especially for the "face" of the NCR in the Mojave.

@"PikaSalt" That or Fallout Louisiana. Though it makes me wonder if we'd get a sort of Point Lookout version of MS or LA.