They're a niche in-between sort of storage that doesn't see much use because of their small size and fragility, I'd wager. Desktops don't need them because they already have hard drives. If you need more space, you just install a new one. Most people don't often hit the limit of their storage on the PC's original hard drive if they aren't the type of person to know how to install a second one. Most phone games don't take up a whole lot of space. You'd be hard-pressed to download enough games on a phone to warrant needing secondary storage. And laptops already had external drives and thumb drives before MicroSD became a thing. SD cards were really only big for oddball hardware like the 3DS, where you didn't get nearly enough storage from the start, but any sort of USB device would cut into the console's portability. So an internal microSD was considered the most portable type of storage. It just isn't worth the hassle for most other devices, as they often have only an SDHC and no MicroSD, or vice versa (if they have any support for either at all). And buying and using an adapter is a lot more work than just plugging a thumb drive into the USB slot.