Why are Shoot-em-ups not more popular?

Out of all video game genres, Shoot-em-ups (or shmups) are probably the most niche of the bunch. Considering the popularity of 3rd and 1st person shooters, you would think that another sub-genre of shooting games would also be popular. Even avid gamers who play a variety of different games probably don't play shmups or know anything about them. Say the names Nintendo, Mario, Pokemon, Zelda, Sega, Sonic, Bungie, Halo, Konami, Metal Gear Solid, Castlevania, Capcom, Street Fighter, SNK, King of Fighters, and a few others, and most people will recognize them. However, I doubt that most gamers would recognize names such a Cave, Toaplan, Progear, Deathsmiles, Raizing, 8ing, Radian Silvergun, Ikaruga, Akai Katana, and others. Is there anyway for the genre to leave the vestiges of obscurity?

Because they're too... monotonous.

Monotonous they may be, but so are fighters and sports games, yet the sell.

Cause it's cool to hate things that are popular today

@jedixman: Does said monotony in your mind expand to the usual subject matter of the genre? If so, I must remind you that not all shmups are space ships and aliens, just like all jrpgs aren't spiky haired anime dudes with huge swords and awesome theme songs.

@fuchsia_nightingale: but they arent popular lol