Because blackout curtrains are more accessible. I live in an apartment and probably couldn't install a rolladen even if I could afford it. You can get good blackout curtains for less than 20 dollars.

I personally never considered them as a solution. It's less preference and more not knowing they existed. I could put them in my current place (and it's tempting to!) but I'm moving again in six to nine months to a place I won't be able to.

Most of the Rollladen I've encountered leak a decent amount of light, so it makes sense to have blackout curtains in front of the Rollladen.