The Shulchan Aruch in Yoreh De'ah 379 outlines a special version of Bentching to be recited in a House of Mourning. I have been in several houses of mourning and have never seen this done. Has anyone else seen it done? Why is it not widely practiced?
Nit'ei Gavriel (Aveilus, vol. 1, 100:3 and footnotes) cites several views on the subject. One is that this version is meant to be said only when there is a minyan for bentching, which doesn't happen very often (except perhaps on Shabbos, but then there other views - cited in the next paragraph - that it shouldn't be recited then anyway). Another is that by rights it should indeed always be said even if there is no mezuman, but that people are negligent about it since it's not a common occurrence.
Kitzur Shulchan Oruch 207 (7) says
that it is not the custom nowadays because we rely on the Poskim who say it was only introduced for bentching with a minyan; very similar to the Nit'ei Gavriel.