A locked door is an obstacle for a cat that prevents it from controlling the entire territory.
What's behind a closed door is inherently very interesting to a cat - especially if there's a human on the other side of the door.
This is why, among other things, cats are so keen to follow us into the bathroom and why they may meow at night behind a closed bedroom door.
While allowing a cat into the bathroom is probably the choice of most pet owners, not everyone wants or can (such as those with allergies) allow a cat into the bedroom at night.
A cat may also meow at the door because she is unhappy that the closed front door does not allow her to explore the environment outside the house.
The following may be especially persistent here:
In all of the situations mentioned, the cat uses meowing at the door as the most effective method of attracting attention and requesting or demanding that the door be opened.