WC isn't dull because it has a story, WC is dull because it has a massively cliche story that's very straightforward in its execution and gives nothing new or interesting to the formula. The story and plot were about as deep as a puddle of water.
The ships are also more or less best described as dull too.
And I've certainly seen classics that had fun gameplay and good story, like FreeSpace 2.
WC had a cliche story, mostly because it created these cliches. You could say that about any sufficiently copied story. It seems to give nothing new or interesting now, simply because it was
the original. TBH, same goes for it's gameplay (you should see the ads for it back in the day...). Keep in mind these things are ancient, and were among the most innovative games
of the day. It still did a few things better, notably damage system and cockpits, which added greatly to immersion. WC3 already had some things we're still missing from FSO.
WC didn't delve into philosophy like FS2 tried to, and IIRC, it was better off for it. It was well written and had good, likeable characters, unlike FS2. BP is how metaphysical subtexts in FS2 should've been handled. I also didn't had anybody to care about in FS2, while in WC, there was a whole ensemble of people whom I really learned to like (and a few I didn't like, but in the "enjoy seeing him clobbered" way

). Also, while the story of most WC games wasn't really deep compared to some others I played, WC4 stands out in that it had a very deep story asking some very important questions. And unlike in the movies, you could choose the answer youself.
Classics are played not for graphics or flashy gameplay, but for deep story, interesting characters and good plot.
Why would I want to play a game with a dull story and dull gameplay? Gameplay is easily the most important aspect of a game, hence the name. A game without good gameplay may as well be a movie or book. 
Actually, Origin even called Wing Commander "Origin Interactive Movie", and the movie-like experience was one of the things I missed the most from FS2. Gameplay in FS2 was also rather medicore compared to I-War or Starshatter, being arcade-y and not consisting of much besides shooting at things (a few gimmick missions notwithstanding). It wasn't much better than in Wing Commander, and even inferior in some places (usually ones important for the immersion).
In general, I think Wing Commander does immersion a lot better than FS2, with little things like cockpits, carrier landings, visible instrument damage, ejection seat, memorable wingmen... FS2 had none of those things.