Today’s modern filesystems were built with spinning-disk hard drives in mind. This is true for Linux’s Ext2/3/4, Windows’ NTFS, and server filesystems like XFS and ZFS. And, of course, so was the original FAT though it wasn’t so much optimised, more simply being one of the first filesystems designed to address magnetic media.
However it’s the simplicity of FAT that makes it attractive for the new generation of storage mediums based on flash memory. Usually, flash memory devices (think SD/memory cards and USB keys) don’t have the fastest interfaces to the computer, and any overhead a filesystem introduces simply slows it down. And, because flash storage devices don’t often approach the volume of spinning-disk drives, you don’t need advanced filesystems to handle them.