With Canonical is developing Mir, a new display server and making it default in Ubuntu 13.10, other Ubuntu derivatives are faced with the difficulty to port their desktop environments to the new display server. As its a lot of work, and the first editions of Mir not that promising, Kubuntu and Lubuntu have decided to drop Mir as their display server in 13.10.
As posted in Lubuntu mailing list:
This is currently an issue for Lubuntu, because, if i understand
correctly, we are going to replace an X + openbox (without composite)
with a Mir + XMir + composite windows manager (?). Considering the
very bad performance on some of our target hardware running under a
composite manager, I’m quite worrying about the performance
difference. The difference between Unity + X and Unity + XMir + Mir
may not be so different, I can’t say the same for Lubuntu.
However, Steve Langasek confirmed on a previous mail that X will
remain for 14.04 for the time frame of the LTS, and I hope it will
still be the case with this change. We currently plan to ship X only
by default for 13.10 and 14.04 for Lubuntu, with eventually an option
to test XMir, to prepare the future.
Lubuntu is targeted for mostly old machines and switching to Mir will decrease its performance. As long as a highly optimized Mir is available, its unlikely that Lubuntu will use Mir by default.