4 of the 8 living PMs showed up for this ceremony. As a parliamentary ceremony it was organized out of the speaker's office so all 8 were invited.

While you are right in your comparison of Harper and Trudeau, my guess is Harper passed because he is the busiest of the bunch in very high demand for his strategic consulting services around the globe.  Plus he was never big on ceremonies.

Presumably, Trudeau and Campbell passed due to other commitments albeit Justin has no genuine love for the institution.

During QP, the speaker introduced the attending PMs sitting in the gallery. 

All 4 got enthusiastic clapping from all sides of the House.

Mulroney got by far the loudest, longest clapping, even cheering, and from all sides, when he stood to be recognized.

I liked that as I liked last week reading a NYT society critic piece saying he'd just heard Brian speak for the 1st time and nobody in modern times but JFK and Reagan were in his class as orators.

At 78, he still impresses.