It's because kommunikation är viktigt is the same thing as det är viktigt med kommunikation.
It may be easier to see in English: communication is important is the same thing as it is important with communication. So the definite form is never used in this case, and viktigt is an adverb rather than an adjective.
I'm confused, having been told in this same exercise that "det finns" tends to be used where the state is more-or-less permanent, and "det är" where the state is more temporary. Isn't the whole point of this sentence that the speaker appears to be anticipating a communication , and therefore the situation is inherently temporary?