I think it's safe to say that mid-90s was when tv entertainment was at its best.
We had The Simpsons, ER, Friends, Picket Fences among other shows. This was the real golden age of tv. Quality shows after quality shows after quality shows. This was when it seemed that everything was possible. For X-Files it kinda was.
Trying to explain why the show was so good is kinda like trying to explain why an apple falls from a tree. That is because the show had a premise that was axiomatic, that is, simple.
X-Files consisted of two likable and relatable main characters, FBI agents Fox Mulder and Dana Scully trying to find out the truth out there. One believer, Mulder, and one sceptic, Scully.
Mulder was convinced that his sister had been abducted by aliens when he was a child. Scully, of course, wasn't convinced. There were other possible explanations, she said. Mulder, however, didn't budge.
This simple but effective setup put them on journey together that won't be easily surpassed.
But that wasn't the only reason the series was so good.
I guess one of the strongest aspects of the show was that not every episode was about the 'plot' that Mulder and Scully were trying to uncover. The seasons also consisting of stand-alone episodes let the writers develope the characters and not rush things too much.
That's one of the reasons that storylines were so well thought out, innovative and fresh. The episodes didn't wear down the audience, which is a problem with so many failed or mediocre shows.
The show also had a wonderful supporting cast. Who could for example forget Skinner, cigarette smoking man, lone gunmen trio, Alex Krycek, Deep Throat, Mulder's father, etc? Just about every character on the show was well thought out and believable, providing humor and drama, depending on the situation.
It was also clear that the writers had done their homework about certain aspects of history that you don't tend to hear about. Mixing the paranormal stuff (fantasy?) with conspiracies (reality) and handling these issues with maturity (or childlike enthusiasm) contributed to the show being such a hit.
I guess one might have thought that the show being about UFOs, paranormal activities, and conspiracies would have made it implausible and far-fetched. That it could have gone all wrong.
In lesser hands this probably would have happened, but fortunately not in the hands of Chris Carter. Here we had (and still have) a guy who was willing to go the distance. Not only was his show ambitious and full of idealism, Carter himself as an invididual raised the bar for others in the industry.
That is because he had the rare ability to take criticism. In fact, he was so good at it, that when an internet poster pointed out what was wrong with a particular episode, Carter not only agreed with the poster but even asked if that person was willing to join the writing staff.
Where else in this industry do you see humility like that?
In the end (I think) we really didn't get to the bottom of the 'alien plot'. Some of those big questions were still left in the air, like the big alien conspiracy that those other 'bad guys' were either trying to abett or trying to prevent from happening.
But I don't think that was really the point of the show. No single reveal or twist or conclusion was something that the audience was desperately waiting to see. X-Files was about characters, humanity and idealism and we as the audience knew that we were in good hands enjoying the ride.