The RAW formats store, well, the raw sensor data from the camera with information on how to decode that for image processors such as Adobe Camera Raw or similar. In that sense, the RAW format is not an image, you have to apply demosaicing algorithms to interpret the sensor data into a coherent image for display. Beyond the Adobe attempt to convince everyone to use DNG (Digital Negative) as their format, there is no standard for the storage of RAW image data and most camera makers have their own. However, what is standard in all RAW formats is the EXIF data which provides a lot of information about the state of the camera settings at the time of the shot.
PNG, on the other hand, was devised initially as an alternative to GIF without all of the patent pain that the latter carried, mostly as a result of the compression algorithm. It's since grown up quite a bit, from original intention, but it wasn't really devised for photography as such. The biggest gap, for example, is the lack of EXIF data, a requirement for camera equipment. The other big issue is that it would be the end result of the interpretation of the sensor data, so you would lose the signal information of the sensor at that point and can no longer re-interpret the information with better or alternative algorithms. Effectively, you become stuck with the interpretation of the developer who wrote the algorithm. That's not alway desired.
So, it really boils down to: different purposes. :)