Reasons for switching to SVGPerfect display - Because SVG is a vector format, the SVGs images are always rendered sharp, regardless on display resolution. For example, they looks good in iPad hi-res displays. PNG images are blurry on such hi-res displays, if you want better results, you need to supply them in higher resolution, which is not a perfect solution (you need to maintain multiple PNG sizes for multiple devices).
But why people are staying with PNG?Compatibility - as PNGs are just plain bitmaps, there is much higher chance they will display properly on various devices. Also, couple of months ago (April 2013), I encountered an Android phone, that could not display the SVG on a webpage, though Chrome for Android should support it (according to the link above).
Performance - SVG is more complicated, probably to takes more system resources to render more complicated image.
No hype - when there HTML5 anc CCS3 was new, there was much discussion how it should be implemented, etc, so people started to use it, just to show off how they are progressive. On the other hand, SVG spec is here already for several years. When the SVG spec was complete, then it was not possible to use it - major browsers did not support it. And nowadays, when you could use it, no one talks about it.
So I would like to start discussion:
Why SVG did not replace PNG yet?
Do you know any sites that use the SVG?
Are you planning to use SVG on your websites?
Check out discussion for this article on Hacker News, that's the most interesting part.