I'd suspect that there are many reasons for it. For me, the primary one is money. My Bluestone plants are just as healthy as anything I can find locally, and cost half as much (small perennials go for about $4.99 each around here, and HD/Lowes don't carry anything I want). Although they aren't known for carrying unique stock, they have a huge variety, at least comparable to the biggest nursery around me. Their service is superb, and their money back guarantee solid. I also mail order roses, most of which are less expensive and healthier than plants I can find locally, and now that I am starting to introduce OGRs, that's probably the only way to get what I want.

Suja

I started ordering from catalogs many, many years ago when I couldn't find the things I wanted locally. As I moved from place to place I continued, again because local nurseries didn't carry the things I was looking for. Now, caring for a seriously ill husband, tho' local nurseries have a wider range than they used to, I shop the internet and from catalogs because I can't leave him. I would guess some people shop in the middle of the night after the kids are asleep, or after a full day at work. Lots of reasons, just as there are lots of gardeners!