Netflix isn't really a recent thing, they started back in 1997 as a subscription service for DVDs by mail. They managed to hang around and then when technology and home internet got better they managed to transition into putting focus on a streaming platform rather than their DVD-by-mail service. They had some pretty humble beginnings so it's only natural that they would focus specifically on America for a while because that is where they stared. I'm not sure at which point they began to spread out, but it likely wasn't until streaming picked up.

As for why they were so slow to go global, this is mostly a matter of licensing restrictions. Usually when you pick up the rights to stream a show, it is limited to specific countries and you have to follow these regulations or else your rights to that program could be revoked.

Also, and this is just an opinion, I don't really think Netflix saw it as a priority for a while. Netflix saw a large increase in customers when they switched to streaming. In particular, I noticed that a lot of people around me started subscribing when the service became available to use on the Nintendo Wii. For a while though, their streaming library wasn't always the best, I would usually get bored and go back to regular TV because I couldn't find anything to watch. But as time went on, they picked up better licenses and that problem of not having anything good to watch faded away. I think Netflix made it their overall priority to provide good content in their home country before expanding to other territories. I believe this is also why they began producing their own shows: to give the viewers something they would enjoy.