Is there a particulare use case that you're looking at? If you give us more detail, we might be able to help more. I don't actually know the specific limitations that make displaying everyone all of the time a constraint. My guess is that it has to do with the client side processing required, but it might be a canvas size limitation. That's sort of assuming that the actual drawing takes place on the client. I haven't looked to see if that's the case. If the rendering takes place server side, then It's probably to keep the rendering down to a manageable number of CPU cycles with hundreds (thousands?) of family trees being rendered at any given time. Of course, all of that is just speculation.
This is what I usually post when people ask about showing as much of the family tree as they can:
Displaying a large and complicated family tree at once is not always possible. Generally speaking, you can always display the ancestors and descendents of any person on the tree, and you can usually display that same information for any siblings of that person.
You can often get what you want, by thoughtfully choosing the person to click on, and changing the options for how many generations of Parents, Children, and Others to show (under the show options link at the bottom of the screen). For instance, if you wanted to show the ancestors of both you and your wife, you could click on your child.