Why is lxqt not available in the trisquel repositories?

Trisquel repository, just like the Ubuntu releases it's based on, don't really update their repos after a release apart from bugfixes.

The current, and soon-to-be-replaced version of Trisquel is based on Ubuntu 14.04. Lxqt wasn't back then what it is today (is Razor-Qt in the repos? I don't recall it being particularly good back in 2014, though. LxQT is a merge between the developers of LXDE and Razor-Qt).

Try following http://ubuntuhandbook.org/index.php/2015/02/install-lxqt-0-9-ubuntu/