According to an Apple release, you won’t see any white iPhones on shelves because they are very hard to manufacture. There have been many thoughts of why the white iPhone will not be here till July, the top most plausible reason that the white iPhones will not be here till July is supply shortage of the materials that make up the white iPhone.
There have also been speculations that the white iPhones need to have their glass painted white and it is hard to find a process to make it a smooth flawless finish. The black iPhones only need to get electroplated while these white iPhones need to go through a whole other process to turn them white. So it seems just to be a slight manufacturing issue that will be solved soon.