Practicing guitar is easy. Not because I find it inherently easier, but because I can pretty much practice whenever I want and won't disturb anyone (myself included). There's a volume knob and while playing and practicing loud is often the most fun, if I can't crank up the volume, I'm not really at a loss. Sure, there may be some tone things I can't do but I can get around to them later when I can crank up the volume.
Practicing singing is a whole different beast. There are so many variables and I can find so many excuses not to practice - I can also find even more reasons to practice, but that's not the point right now.
If there are people around, I won't practice because I don't want to be loud and bother them. If it's too early, I won't practice because it's not conducive to good practicing (although if I have to practice early or not practice at all, I'll take the former). If it's too late, see reason number one. If I'm sick, I probably won't practice. If I'm tired, I probably won't practice. I sound like a petulant child right now. Reading this makes me want to go practice - which I was going to do anyway.
The reasons why I should (and want to) practice singing are much longer. Too long for me to get into right now because I want to go practice.