I don't believe the file size of .pngs is worth having transperency. png-24 has a large color pallette much like jpegs and has transperency but the file size is nearly two times a 100% quality jpeg. Not worth it imo. png-8 has only 256 color limit like a gif and transperancy aswell but its also bigger then a gif with the exact same colors. It also doesn't support animation. 
Erm, it's because PNGs are lossless, unlike JPEGs which are lossy. It's basically a lossless format without the 256-colour limitation that plagues GIFs. I'm not saying PNGs are right for everything, but they do fill a certain niche. There's MNGs if you want animation.