Jerry Hildenbrand for Android Central:
… Prior to Android 4.4 KitKat, applications — provided they had permission to access the SD card — could read and write to any area on removable storage, including the system folders like DCIM, Alarms, etc. That has all changed, and now third-party applications — as in ones you download from Google Play or elsewhere — can only write to files and folders that they have created or have taken ownership of.
This keeps things “tidy.” Apps aren’t dumping files everywhere on the card — something we’ve all encountered — and instead have one central location to put all their files. There also are some serious security concerns that were addressed by not letting an app write files just anywhere.
Gee, what a surprise. In pursuit of security, Google has to move toward a file model that iOS used from the start. This is not without significant drawbacks and frustrations, but apparently Apple’s engineers figured out what compromise would be best for security and stability a long time ago, even though they weren’t dealing with removable cross-platform media.