I don't know the _motivation_ for why it leaks, but it leaks because the method calls of a context manager are __enter__ and __exit__, but not __del__. [0]
The motivation is especially opaque, because though CPython does run __exit__ immediately, it isn't guaranteed to do so by the standard. So __exit__ will happen at some point, like __del__ with the GC, but it won't actually call __del__, which makes GC'ing the value more complicated as __del__ can't safely run until __exit__ does.
I _think_ the motivation is something to do with __exit__ being allowed to throw an exception that can be caught, whereas __del__ isn't allowed to do that, but I don't find it that convincing. And I've never seen an example where the introduced variable is ever referenced afterwards in Python's documentation (including in the PEP itself).