Besides the reasons given by Yannis Rizos' answer, this won't happen since they are not identical (despite looking so at first glance): Ask Ubuntu is for Ubuntu-specific questions (using e.g. Unity and all its features, PPAs, Launchpad etc... - asking questions about other distros is disallowed, and will be migrated to Unix & Linux or closed) and Unix & Linux is for general UNIX/Linux questions which might be not distro-specific at all (e.g. awk and sed questions).
As for the differences between Unix & Linux and Ask Ubuntu, gilles sums it up nicely in his answer to this question: Ask Ubuntu, version 2.0.
Ubuntu sets up a core environment that most Ubuntu users stick to. That makes Ubuntu different from most other Linux distributions. Ask Ubuntu allows answerers to provide an Ubuntu-specific perspective to all questions, which a tag on a wider site is not quite as conducive to. There is a difference in treatment between ubuntu-tagged questions on Unix & Linux and questions on Ask Ubuntu. Ubuntu users who don't care that there are other Linux variants — which is the majority of them — get more useful answers this way.