Modern bicycle retailers have high overhead costs and lots of capital tied up in inventory. They are strong-armed by bicycle vendors into very heavy pre-season orders which minimize their open-to-buy capital. You can't get the margins you need unless you order heavy and deep, and if the vendor thinks your order is still light, they'll talk about opening up another dealer nearby. It's hard enough to order/stock enough of the parts/accessories that sell in quantity, there is little money left for things that sell in very small quantities. Then they face considerable price competition from online vendors on all parts, accessory and clothing items. Most can't afford to stock items that are primarily of interest to classic & vintage fans. It's also difficult to find/afford seasoned staff that knows about freewheels and dt shifters and etc, etc, yadda yadda yadda.

There are clearly exceptions to this, and none of this excuses poor customer service, but I can understand why most shops can't/don't stock a variety of vintage-specific product. I don't think less of them for it, and I do value those that still keep our flame burning.