Why there is no easy way to install new themes in ubuntu 11.10 and 12.04 in both unity and gnome 3 ?
You might be referring to Gnome Shell, but if you're talking about window decorators and the contents of windows, then Unity and Gnome Shell aren't relevant. They're both Gnome 3.
The reason is that lots of the infrastructure has been replaced. Old Gnome themes are no longer compatible. That means it'll take a little while for new themes to become available to install. The configuration system has also been replaced, so the graphical tools to do these things must also be replaced. Since there's relatively few themes to choose between, that task probably hasn't had the highest priority. It'll improve. :)
I would expect to install themes from the software centre, like any other software. Let's hope we'll get lots of stunning ones very soon.