Probably cheap.

I bought one from China (was about the cost of a loaf of bread). Thin strip of red LEDs as thin as a credit card. What's not to like?

Well, the gel filter strip was red, yeah. But the diodes underneath were white. And not just any white, but a cold blue/white.

The thing about colour filtration is you can only let red pass through if there's any red light to actually pass through... but these LEDs don't work like glowing hot metal, so it doesn't matter how much red filtration you cover them with, any direct light making it through is still cold - usually an off-purplish colour light, with only the filter material itself glowing nearer it's colour by the non-directional light.