Because a big part of the point of this is to get rid of the legacy cruft that is in the X11 protocol.
> As is, if i'm not wrong, it is a choise between Wayland apps or X apps.
You are wrong. You can have an X server which renders to Wayland.
> The idea of a server on top of another server, seems more clumsy and ugly hack than most things that can be said about xorg "codebase".
Allowing multiple processes to work in concert to provide some service is a fine and potentially elegant idea. An X11 server which works with the Wayland server is not importantly different from a window manager which works with the X11 server. In each case, different tasks are taken on by different processes.
Did you know that some apache web servers serve content that is stored in an NFS filesystem? This can be quite effective and again is not inherently different from an Xorg/Wayland combination.
Certainly there is a large risk in developing wayland if the goal is to replace X11 everywhere. However if the goal is to have fun, learn stuff, and create something that might be useful, then the risk isn't all that great.