Why wasn't Silpheed well liked on the PS2?

[Paratech]

Was it too easy? too short? overshadowed by Gradius 3/4 or other shooters?
I just got my PS2 last night and it was one of the games I previously purchased for it. I was "wowed" by the graphics and thought the game was "playable" unlike a few other shooters that come to mind.
I also thought the graphics were good and the game was fun. I suck at shooters, so maybe I can't breeze through it like others, but I'm already fond of this game. It's definitely worth the $7.50 I paid for it...
So what am I missing? How is it mediocre?

[Taisen]

There wasn't a modicum of interest in shmups this generation until Ikaruga. Gradius 3/4 did not overshadow Silpheed. They were both unpopular early ps2 titles. Gamers didn't buy the ps2 for 2D gaming.

[Allnatural]

Most reviews cite the unusual isometric (rather than top-down) perspective and the lackluster weapons. I tend to agree, but I still like it. It's no Gradius V or Castle Shikigami 2 though.