Are there any technical reasons that prevent WP to be part of Framework Interop Group?
Yes, WordPress’s maintains backward compatibility (spaghetti), it would not be feasible to adhere to PSR-0 and rename everything, which is somewhat unfortunate for the long-term, with regards to FIG.
That being said it’s possible to adopt it for plugins and themes, there is a trac about it here:
https://core.trac.wordpress.org/ticket/21300