PS definitely IS the black sheep in Cave's catalog. At the recent Cave festival it was the only recent game that wasn't on display, for example, and as you mentioned it's the only recent Cave game not to get a superplay release (though MMP didn't get a proper one either).
The problem with PS is 1)it's ridiculously difficult, 2) you basically have to break the game (max out rank, get infinite lives) if you want to score well, 3) you have to milk everything for about 80minutes (seriously, that's how long a 1 credit clear lasts with full milking), but the milking is basically just ticking away with your shot on midbosses and bosses, and 4)it's buggy and can randomly freeze in the later levels. Hardly a formula that appeals to most shooting fans.