There is no (known) selection effect that would account for that gap. (And scientists have looked to find such selection effects). And yes, there is other gaps,

There is no (known) selection effect that would account for that gap. (And scientists have looked to find such selection effects). And yes, there is other gaps, e.g. a lack of systems with a planet at 0.2 AU (or rather in the range from 0.1 to 1.0 AU, separating the "hot jupiters" far in from the "normal" gas giants further out that we know from our solar system).