At the risk of sounding a bit like Chronobound here, I think I'll take a shot at answering this.
There's the whole fact of it being the first game on the GBA. And while I do question the point Chronobound occasionally made about it being a revival of the series or a celebration of some sort, there's a kernel of truth to those claims. The game was the first released after Thracia 776(which was a SFC game released in 1999, for pete's sake), and it does distil elements from all the previous games(though I'm not sure that qualifies as a "celebration" as much as a "hey let's take a look at all the mechanics and ideas we introduced over the last two games and see which ones work the best").
The game also got what seems to me to be a lot of promotion when it was released. There was Roy's appearance in Smash before his game was released obv. There's also Hasha no Tsurugi, which was a loose adaptation/expansion of the game's plot in manga form. Of course, most of the other games in the series got manga adaptations as well, but HnT is notable in that, to my knowledge, it's the only one of the manga adaptations to be referenced within the game it's adapting. The main characters of the manga are referenced via special named weapons(Al's Sword, Gant's Lance, Tiena's Staff) that can be obtained through the campaign. Additionally, one of the bonus trial maps avaliable at a download event(
yes, apparently all the GBA games had Pokemon-style download events) was based on a chapter in the manga where Al and Gant rescue Tiena.
I'm just spit balling some ideas here. I honestly have no idea if what Chronobound says about the game being seen as 'nostalgic' is true or not, but if he is right, these might be some of the reasons.
(oh, and fyi some of the 'evidence' Chronobound occasionally used for why FE6 was supposedly such a big deal was total nonsense, like claiming that the game having some rearranged tunes from older FE games means it's some sort of series celebration)