Why Is Star Trek The Huge Success That It Is?

I was going over youtube’s many entries re: various star trek series, films, the new film coming out, even fan based created shorts complete with film footage, home made special fx, and authentic looking uniforms, props and sets. Star Trek has become a sub-genre all its own within the larger framework of sci fi in general. Tell in your own words what you love about Star Trek, and why you think its been so successful and more than 40 yrs in longevity.