Expanding on Kilian Foth's answer, this direction of layering corresponds to a direction in which a human explores a system.
Imagine that you are a new developer tasked with fixing a bug in the layered system.
Bugs are usually a mismatch between what customer needs and what he gets. As the customer communicates with the system through UI, and gets result through UI (UI literally means 'user interface'), the bugs are reported in terms of UI too. So, as a developer, you don't have much choice but to start looking at UI too, to figure out what happened.
That is why having top-down layer connections is necessary. Now, why don't we have connections going both ways?
Well, you have three scenarios how that bug could ever occur.
It could occur in UI code itself, and so be localised there. This is easy, you just need to find a place and fix it.
It could occur in other parts of the system as a result of calls made from UI. Which is moderately difficult, you trace a tree of calls, find a place where the error occurs, and fix it.
And it could occur as a result of a call INTO your UI code. Which is hard, you have to catch the call, find its source, then figure out where the error occurs. Considering that a point you start at is situated deep down in a single branch of a call tree, AND you need to find a correct call tree first, there could be several calls into the UI code, you have your debugging cut out for you.
To eliminate the hardest case as much as possible, the circular dependencies are strongly discouraged, layers connect mostly in top-down fashion. Even when a connection the other way is needed, it is usually limited and clearly defined. For example, even with callbacks, which are a sort of reverse connection, the code being called in callback usually provides this callback in the first place, implementing a sort of "opt-in" for reverse connections, and limiting their impact on understanding a system.
Layering is a tool, and primarily aimed at developers supporting an existing system. Well, connections between layers reflect that, too.