Facebook had image uploading via mobile at the same time instagram was purchased. So it isn't just that.
Instagram smartest thing they did was leverage the Facebook social graph (something much harder to do now) to integrate instagram photos as posts in the feed, so that when a user clicked on them, you ended up at instagram rather than staying inside Facebook.
Instagram was a camera + profile app. Snapchat was a camera+message.
Facebook took a long time to get mobile right. Instagrams simplicity, and Facebooks lack of response was what allowed Instagram to grow. And dont forget Instagram was originally more a photo hosting adoon for twitter, similar to imgur to reddit.
Instagram smartest thing they did was leverage the Facebook social graph (something much harder to do now) to integrate instagram photos as posts in the feed, so that when a user clicked on them, you ended up at instagram rather than staying inside Facebook.