Well, the licensing is just speculation. While SVG is open, that’s not the same as ‘anyone can use the code’. Many open licences have restrictions on who can use something for free, and most often they are more restrictive the bigger the company is that want to use it. I don’t know. I just threw it out there, because there have been apps removed from the App Store because of open license issues (counter intuitive as that sounds). And that’s just an app. I imagine some licenses get even more restrictive when it comes to building the code into an OS.
More likely, it’s just that Apple considers the ability of third party apps to support SVG as sufficient. I have three vector graphics apps that support SVG. This lets me move projects around, and realy, that’s all SVG was designed for; a open format that lets you move vector graphic projects between apps and platforms. It’s got no benifits over other formats when it comes to end user image assets and libraries.
As for it not taking off? Well, being open is not a ticket to instant success. Sometimes open wins, and sometimes it doesn’t. More often it sticks around as an alternative for those willing to trade a little extra work in order to avoid paying the big guy’s on the block.