Mostly its because it tells a complete story. Most series begin when someone pitches a decent idea for a show... and then it gets written by committee until its quality completely degrades and its canceled. B5 was different. B5 was a completely storyboarded concept from start to finish, with the episode writing just filling in the blanks between major plot lines. And those blanks were filled in by a talented writing staff. So unlike most shows B5 starts pretty good... and just gets better and better.
Now, it outlived its end... that's a problem. Anyone watching it can leave it at season 5 and really only miss about 1 and a half good episodes. But, by that time most people love it so much they will watch that last season anyway.
What I liked about it, it was like Star Trek... except its mix of humor into the drama was much more well done and the humor was at times pretty dark (Vir and his "little wave" comes to mind.)
And I rewatched it recently. Its effects really aren't that bad, depending on expectations. I'm one of those people who expects that after the bombastic splendor people see on tv and in movies... the reality of space travel and the technology level surrounding it will probably be a little underwhelming visually anyway. It just adds to the "tin can in space" realistic atmosphere as opposed to Star Trek's sterile "still the safest way to travel, let's put our children on the ship" atmosphere.
It is a little difficult to compare it to other sci-fi though. In a lot of ways it is a little too unique for a comparison to be descriptive. Its a political drama, but the races involved have their parallels to real societies jumbled up to lessen verisimilitude. There's an empire on its last legs... maybe Austria/Hungary before WW1, kind of. There's a race on the rise that had been until recently subjugated, and are over ambitious and warlike but have a surprisingly zen religion that only some follow... parallels to Israel and Palestine there really. There's a pretty superior race that is very zenophobic with a rigid religion and caste system, a kind of indo-Chinese mix with tech far advanced maybe. And another VERY advanced race that is mysterious and near completely isolationist... Wakanda-ish? And that's just the alien races. Humans are pretty split between Marsies and Earthers and are the backwater bumpkins of the galaxy who until recently could just barely travel beyond their own star system. The humans are finishing rebuilding after a war where they got their teeth kicked in, very nearly genocided into extinction over a misunderstood first contact with a faaarrr superior race.