Your welcome. I'll be a bit more specific about why.
First off, usb drives or flash cards or thumb drives...whatever you want to call them use pretty much the same technology as SSDs do however the Nand memory in usb drives is much much cheaper, lower quality, and therefore slower. It also operates on less channels. In addition, dont forget interface...USB is extremenly slow...its theoretical limit for usb 2.0 is like 60MB per second vs SATA II which can do like 375MB as a theoretical limit so big difference there.