Why doesn't the Horsehead Nebula glow like other nebulae?

Horsehead Nebula doesn't glow like other nebulae because it is older than other nebulae and was probably a lot smaller. In addition, Horsehead Nebula is farther away from the Earth than other nebulae, so it may seem that it doesn't glow as bright as other nebulae.

No, this isn't right!

The horsehead nebula doesn't glow for one reason - it isn't illuminated by any stars whose energy is enough to make the gas shine. That's the only difference between a 'bright' diffuse nebula (like M42) and a 'dark' one like the Horsehead or the Coalsack. M42 (the Orion nebula) is bright because there are several hot, energetic stars embedded inside it, and which cause it to 'shine'.