Bolt's improvement in performance can be accredited largely to the fact that he is still maturing as a human. The kid is only 23 years old. Males do not typically reach peak muscular maturity until they are 25. We can expect him to improve for another couple of years, at least.
Another fact that gives him an advantage is his height of 6'5". Running speed is determined by two factors: stride length and stride frequency. Sprinters train specifically to improve both of these aspects of performance. with all other factors being roughly equal among top sprinters (access to coaching, stride frequency, nutrition, shoes, etc.) it makes sense that Bolt is dominant.
It was only a matter of time before a sprinter of his height and with the biomechanical structure (origin and insertion points of quads, hams, glutes, and calves), and muscle fiber composition (predominately glycolytic white fibers) would come along and smash a lot of records.