They are in very short supply! Hermes is a bit weird about them. You can call a store to see what inventory they have in stock, but you can’t purchase over the phone or have them shipped to you. You need to physically buy the Birkin in store (though I imagine many fancy people have assistants to do this). For some colors there is a super long wait list. I had read a while back that increasing costs for raising cattle had increased the cost of leather, making designer goods more expensive. That might apply here too, but mainly I think the issue is that these handbags are in such short supply that the easiest way to get a color/leather combo you want is to buy it on the secondary market, which drives up costs.