My first bub didn't walk til 18months, and my second only recently started standing supported, maybe at 12 months? I found it's just because, being a second child, I don't have the time to encourage him to stand all the time. When I realised he wasn't standing I just would pull him to his feet when I was sitting, a bit at time - now he's standing just fine and climbing things.
Um...what's the moral of my story...oh yeah, that maybe if you just persist in getting bub to stand supported, then her legs will get stronger.