For the most part, I would guess that I, along with most of the other feed providers, really don't have much of a way of scheduling when our feeds will be up or down. For me, I only have the one handheld, so if I want to use it for something other than air traffic control, I have to take it offline to do so.
Also in order to keep the feeds up, people must have their computers on at all times. I can probably speak for everyone in saying this isn't always possible.
There are many reasons a feed may be down. We all (the feed providers) know that people enjoy the feeds, and we try to have them up as much as possible. However, it's hard to gauge when the feeds will be down and it's hard to keep them up 24/7. We all try the best we can.
There are some feeds I enjoy that are down periodically. Thus, absence makes the heart grow fonder...it's that much better when it is up!
Thanks for understanding.