Usually, the firmware in a USB flash drive isn't as good at wear leveling as a full fledged SSD. Many flash drives are also "optimized" to work only with MS-DOS, MS-Win, and OS/X filesystems; they can perform rather poorly when used for anything else. They also tend to be much, MUCH slower at writing than a real SSD, and usually slower at reading as well.

A lot of that comes from the grade and quantity of flash chips used. An SSD has a lot more space for chips, and so normally implements many parallel channels from which higher speeds can be derived. USB sticks usually have as few chips as possible, to keep the form factor small, and often use a very low grade of chip that wears out 10-1000X faster than an SSD.

Cheers