First of all, there actually is a page on the Ask Ubuntu help center directing users here. This is a quote from AU's "on topic" help page (emphasis mine):
Questions that you should avoid:
Linux Mint, Elementary OS, Backtrack, Gnome-Remix (prior to 13.04) and other Linux distributions (try our friends at Unix & Linux Stack Exchange).
However, while the scopes of the two sites most certainly do overlap, they are not identical. We deal with all *nix systems while AU only deals with Ubuntu flavors. Furthermore, AU covers services provided by Canonical such as Launchpad, Ubuntu One, Ubuntu Advantage and anything else offered by Canonical.
In addition, the majority of Ubuntu users use Unity, a desktop environment that is almost exclusively used by Ubuntu users and, therefore, questions about Unity, while perfectly on topic here, are unlikely to find an answer.
In short, while the scopes of the two sites clearly overlap, they are not identical. Some things that are on topic on AU are not on topic here and vice versa. More importantly, the character of the two sites is quite different. We're much more command line oriented while AU focuses most often on GUI approaches. The users of AU do not care about portability issues or other non-Ubuntu OSs. Also, since AU is actually officially recognized as a support portal by Canonical, AU gets loads of questions from users who have no idea how the SE model works. I can assure you that the U&L users have no interest in receiving those questions.
Finally, yes there are command line questions asked on AU instead of here. However, shell scripting is on topic on AU, U&L, Stack Overflow, Super User, Ask Different and probably others. We can't force people to post them here just because they're also on topic here.
So, the sites will not be merged since they cater to different audiences.