When I go into the Dark Brotherhood's sanctuary, my followers do not come in with me?

This is the first time I'm being with the Dark Brotherhood. When I go into their home my followers do not come with me? Why is that?

Solution 1:

Only you are part of the dark brotherhood, not the followers you bring. You may be able to have them come inside once you become higher rank in the Dark Brotherhood, as I do believe in the second sanctuary your followers can come inside.

Solution 2:

Because they'd have to die if they were to come in. It is the Dark Brotherhood, after all.

Solution 3:

At the sanctuary near Falkreath they cannot enter, but in the Dawnstar sanctuary they can.