I have worked in sales a little in my life, but never with clothes, so maybe this is not relevant or correct.
I find that lots of places simply don’t keep stock anymore. Keeping stock ties up a lot of your money, stock and staff are always going to be your main costs in sales. Because of this, I find that many companies will avoid stock wherever possible, with the goods going from the factory directly to the customers doors (if/when delivered).
Lets say you order a shirt, a real expensive one, like $300 bucks or something. If each sale earns the shop $40, with the rest going on paying the maker and costs, and you want to order one, and there is a $50 delivery cost for the store, then selling you a shirt becomes a loss.
Maybe they are friendly with the delivery companies, and they could just deliver 1 extra shirt for free. Even if that is the case, they could lose the shirt on the way, and you could turn in to a complaint, and complaints cost money because you need staff and forms.
If you go to a small shop, there is a higher chance they can deliver, but if you go to a big chain store, they have teenage staff who are paid nothing, who have to follow a very well defined set of rules and procedures, and if you ask them anything outside of the rules or procedures, it’s always going to be a no.