uudecode on some systems cannot write to stdout, but tramp depends on encoding and decoding programs to be able to read from stdin and write to stdout.
We can find ways to circumvent uudecode's ability to write to stdout, such as writing to a temporary file and then piping that to stdout.
But I have decided not to implement workarounds as they are too fragile to work reliably. Some on systems, tramp will not have uuencode method.
The Emacs maintainers wish to use a unified file name syntax for Ange-FTP and tramp so that users don't have to learn yet another syntax though it is okay to learn new extensions.
For the XEmacs maintainers, the disruption from a unified file name syntax are not worth the gains. Firstly, the XEmacs package system relies on EFS for downloading new packages and therefore is already installed. On the other hand, tramp is not installed by default in XEmacs. Unifying will require tramp installed from the start.