Two inhibiting factors that control the progression of native species are disease and insects. An exotic species brought into an area comes with no such controls. Hence, exotic plants have a distinct advantage over the native species. If an exotic species takes hold and extends its range, thus overshadowing or eliminating the native species, the result will be a disruption of the biological interactions. Again, kudzu is an example of an exotic species which has literally taken over miles of terrain because the limiting factors do not exist.
Nothing eats it.
It does not seem to be susceptible to disease. Therefore, it continues to spread. I have even heard that one southern state has discovered that domestic goats will eat kudzu. This state, taking this factor into account, is experimenting with the idea of renting goats to try to control the progression of this species. Native species, on the other hand, have the limiting factors in place.