How come some apps can't be written to the SD card? A question that has bugged me is why app developers (especially Google) do not allow apps to be written to the SD card.  I understand there are a few that cannot; however, I think there are still quite a few apps that can run from SD, which would benefit those of us with small amounts of internal memory.
Here is an excellent list of characteristics of apps which should not be installed on SD card, with reasons, from the Android developer site. Hope it helps.