An apostate is one who leaves the JW religion
That is true for any religion, not just the Watchtower Society. The Watchtower's door-to-door work is intended to convince householders to become apostates from whatever other religion they hold.
and purposely tries to go out of there way to stop others from learning the truth.
That has nothing to do with the meaning of "apostate." That is the opinion of the Watchtower Society; it is an unjustified hyperbole used to impugn the character of those who leave their organization.
Why do so many think JW Religion is a Cult?
I prefer to say "high control group." The word "cult" has negative connotations that polarize the discussion and bring meaningful communication to a standstill (this thread is an example of that effect.) Those who have endured bitter experiences with the Watchtower's effect on their family, their friends and their own life often refer to the organization using strong and disparaging language; that is human nature. Frankly, "cult" is a rather benign word to describe the root growing the bitter fruit so many former Witnesses have endured.