Powers of two... add an extra address line and that's what you get.
For a whole lot of reasons, a full address space makes life easier. For the chip makers it's pretty much essential otherwise you couldn't make a bank of arbitrary size without holes in it.
And the flash drive market pretty much follows the chips. This doesn't happen with spinning metal.
You *could* create a 3GB flash drive from three 1GB chips, but you wouldn't do it if 2GB chips were available because generally speaking a 2GB chip will cost less than two 1GB chips. So economic forces tend to take you along the line of flash storage being a power of two multiple of the chip size, and consequently a power of two.