why no upgrade to 1.6.8 yet?

Perhaps no one has gotten around to it. Or maybe a number of stability fixes from 1.6.7 to 1.6.8 have been previously applied to the Ubuntu version of lyx.

If you believe that lyx in Ubuntu should be updated from 1.6.7 to 1.6.8, I recommend reading https://help.ubuntu.com/community/ReportingBugs carefully, and then filing a bug report / feature request for it to be updated. In your bug, you should make sure to explain why it is that you believe that lyx should be updated to version 1.6.8. I am guessing that your reasoning is that 1.6.8 is a stability release intended to ensure that the 1.6 line is rock-solid before transitioning all development to the 2.0 line, so 1.6.8 is the ideal 1.6 series version to use. Whatever your argument as to why 1.6.8 should be provided in Ubuntu, you should articulate it in the bug you file.

If you file a bug, then after doing so, you can use the "Link existing bug" link in this question to link to the bug from this question. This will also put a link to this question in the bug. However, your bug report should be self-contained--developers should not have to view this question in order to understand and appreciate your report.

In the mean time, there don't appear to be any PPA's providing lyx 1.6.8 for Ubuntu, but you could build and install that version from source if you wish. If you want help doing that, please feel free to open a question about it. (You'd probably have better luck starting a new question, since that is a related, but definitely different, topic.)