What makes you say that, though? I mean, there are almost nothing but weighty shmups on XBLA. Ikaruga, RSG, Guwange, Triggerheart, Sine Mora, Space Invaders IG, R-Type...
The only ones up there I'd compare in quantity of overall content are the ones that have been ported from previous console-releases. And even then, they don't all feel as if they have as much content as TWN, really.
TWN! has full screen character art for each main character, their familiar, and the bosses, drawn in the HD-style, and the original PC art style, for example, which also show different emotions in the art. All these character encounters are fully voiced, in 2 different languages. Along with this, it has different story and voicework for arcade and story modes, as well as multi-image openings and endings in the story mode, for each character.
It just feels like a higher-class port than many others. It feels more like an arcade conversion with all the console-friendly bells-and-whistles added, rather than a straight arcade port, where little was done to add value.
TWN also had an unlockable character if I remember right, and a cheap (like 2 bucks?) DLC character, each of which were treated to the full set of options (Seperate leaderboards for bonus modes, arcade and story stuff), and didn't feel like shoe-horned in bonuses, which got any less attention than the main cast.
Of the games you listed up there, I'd say it's more in line with the content Sine Mora shows (with it's deep story and arcade modes, as well as an unlock like it's alt story), rather than, say, Triggerheart, or Space Invaders. TH feels like a simple, bare-bones arcade expereience, and SI's minimalistic graphics / presentation is the exact kind of thing that screams "Made specificially for downloads and quick consumption!" to me. Not that there's anything WRONG with that, but it doesn't make me think "Man, I would have been totally happy paying 30/40 bucks for this on PS2!", like Sine Mora or TWN! do.