From a slightly different angle: hard drives are inherently analog, when the magnetic fields write data onto the disk. There's no particular reason for any particular data size to exist; it's just a basis of how much physical room they have, and how tightly they can write and reliably read their magnetic fields. They're typically trading off speed versus size versus reliability versus cost; improving any one tends to weaken the other three. They're trying to juggle those three competing priorities, and then come up with products that humans will buy, and they end up going for round numbers to make marketing easier. (ie, 1TB was a really big deal; 980 megabytes is essentially the same thing, but a terabyte is a mental barrier. 980-meg drives wouldn't sell nearly as well as drives just 20 megabytes bigger.)

With memory-based devices, everything is built with transistors, which are on/off devices accessed over a specific kind of data bus. Because they're firmly in the digital domain, they work based on powers of 2. Basically, SSD manufacturers buy memory chips, each of which hold some power-of-two amount, and then group up those chips into a coherent device that looks like a disk drive. For best speed and efficiency (lower cost) they tend to want to fully populate memory buses, so the capacity of a flash drive will nearly always be related to the power of 2 in some way.

The tl;dr: Hard drives are analog, trying to squeeze as many magnetic marks onto a spinning platter as they can. They can be powers-of-anything, since that's not related to how they work or how much space is available. Powers of ten happen to be easy to sell.

Flash drives, being additively constructed, and addressed with digital buses, are easiest to make using powers-of-two.