I guess because it has 5 movements rather than 4. The first use of the idée fixe? Anyway, this piece isn’t a favorite of mine. Why not listen to Brahms? I downloaded the Muti version and this work still doesn’t do it.
It’s no favourite of mine, either. But it’s a crowd-pleaser with a proved track record, which counts for a lot as far as reputation goes. As for Berlioz in general, his influence on later composers, in particular leading to Wagner, is perhaps more important than the actual music he left us, but pieces such as the Symphonie Fantastique are still good examples of fairly early romanticism in a purely instrumental form.
I don’t think Fantastique is meant to imply anything in terms of the excellence of the work, but it is telling the story of a dream or fantasy - an artist meets a beautiful young lady, they dance at a ball, he woos her and is rejected whereupon he murders her, he is dragged to the guillotine and sees a vision of her the instant before the blade falls, and his spirit is tormented by her ghost and others in a graveyard sabbat.
IIRC, the movements of the symphony represent the visions of a composer during an opium-induced ecstasy; the first three movements are an imagined reality, the last two an imagined nightmare. The fifth and final movement heads, after a brief introduction and a final appearance of the idée fixe, into the Dies irae, followed by the Sabbath of the Witches; both meet to culminate into a conclusion.
If you were asking why it has that name, Malacandra and wintertime answered it. But it sounds like you were asking if we could account for the popularity and supposed importance of the work.
I think the “program” or story that goes along with it accounts for a lot of its popularity among the general public. It appeals to the imagination and makes it a lot easier to talk or write about if you’re a music popularizer, compared to a Brahms symphony.
It’s a successor of sorts to Beethoven’s Pastoral (6th) symphony, and a key piece in the development of program music, which was the direction that some music went during the Romantic period (and Brahms is pretty much the epitome of the music that didn’t go in that direction).
I was pretty meh on the piece after hearing at least two or three different recordings of it, until I came across a version that clicked for me (the one by Myung-Whun Chung and the Orchestre de L’Opera Bastille) that made me think, “Hey! This is actually good stuff!”
I suspect that the controversy surrounding the use of the Dies irae in the Witch’s Sabbath movement probably helped both boost and maintain its popularity.
For me Symphonie Fantastique is one of those big leaps forward in music on a par with The Rite Of Spring. The sound and emotional upheaval in that piece anticipate a lot that didn’t show up in other composers’ work for many years. Berlioz wrote other music that was startling for its time (including the overture to Le Corsaire) but this symphony was a fantastic achievement.
Possibly some look down on it because it’s “program music” and parts have found their way into movies when someone wants to create a sense of dread (The Shining, Sleeping With The Enemy).
One favorite part for me (at the culmination of the march to the scaffold sequence) is when the inevitable is about to happen, that whiny little woodwind comes in to plead its case, and…nope, sorry about that. 
What I love about the piece is that it went way beyond how previous composers expressed specific emotions in telling a story. This had really never been done so successfully. And Berlioz was a master of orchestration; throughout the piece I have the feeling that he devoted a lot of effort choosing the exact instruments to express specific feelings. In this respect I put Berlioz in the same category as Rimsky-Korsakov.
I put them in the same category as each other, too, but I think it might be a different one 
In other words, I’m more of the anti-programmatic mindset, in that I regard resorting to such an extra-musical element in order to dictate to the listener the expected emotional reaction to be a cheap and easy way out, and one which precludes true communication on the most instinctive levels.
I agree. I was only comparing them in regard to their superlative orchestration skills.
Yeah, as others have said, it’s really considered a milestone in the development of program music. I’m mostly “meh” about the piece as a whole but I think the 5th movement is quite brilliant. Once you take a close look at how the program notes match up with the music I think a very visual story comes to life - and it’s dark and disturbing and even kinda hot (if you’re into that sort of thing )
And GM, I don’t see program notes as a way to dictate to a listener what they’re supposed to feel, but rather as inextricably entwined in the piece of art. In Berlioz’s SF I don’t believe the program notes do anything more than give you some info on characters and plot (and very little info at that), elements which are not possible to communicate solely through music.
That’s pretty much the way I see it too (though I have deliberately cut the quote before you started to …, well, Moe, ewwh … ;))
But unlike Thudlow Boink, I haven’t found a recording yet that sounds as good as the 5th movement appears on paper. I should get a hold of the one he mentioned.
Btw, does anyone know if Harriet Smithson was ever offended by the way Berlioz integrated her into the final movement? Yes, I know, she married him later, but, IIRC, he was furious with her during the time he wrote the movement.