It was not an obvious idea at the time, simply because there weren't that many people on the web. It was around the time that SSL was being developed [1] -- so if you were much earlier you probably couldn't take payments securely.
There were probably mail order catalogs that had websites, but I think that is a different thing than a "web store".
Also, Netflix wasn't the first online movie portal, but I'm pretty sure they were the first people to make a business out of mailing DVDs! It's crazy that they started like that.
[1] https://en.wikipedia.org/wiki/Transport_Layer_Security#SSL_1...