I think because at night they are asleep. During the day they are running and playing and jogging their bladders ...not a scientific answer but it's my guesstimate.
I think closer to a year when their bladder as well as their body is fully grown you can let him/her loose to run around. Zoey is 9 months now and when I am home I constantly take her out..but she will hold for good stretches. I think it has to do with them being ornery as well
Also I think males tend to do a lot better if you neuter them early. That way they don't get into the marking of things.