I seem to remember Phil Hersh tweeting about empty seats in London. This article also talks about the empty seats from the first 3 days
http://www.lfpress.com/2013/03/15/t...-world-figure-skating-championships-in-london
I think the London worlds sold decently and they did likely sell out for the dance and women final, but the arena was only configured to seat 7,000. I have a feeling Skate Canada is probably not keen on having worlds in an NHL arena again after what happened at 2006 worlds in Calgary. I found an archived article which said total attendance was 94,000 for that event (which would have included more sessions than it does today because of the qualifying round) and said that most sessions were "half empty". From what I remember of the event on TV, half empty seems like an exaggeration, but maybe others who were there can comment.
I think that popularity in Canada is in a better place today than it was in 2006, but they probably aren't confident they'd get a respectable sized crowd in a larger venue. For the most part they've stayed away from venues larger than 10,000 for all their events since then with a handful of exceptions.
They used the Canadian Tire Centre in Ottawa which seats 19,000 for 2008 Skate Canada and 2014 Nationals, and the Colisée in Quebec City for 2007 Skate Canada which seated 15,000. The Pacific Coliseum was used for 2008 Nationals and 2009 4CC, but this likely was chosen because it was the olympic venue. Other than that, all the events have been in small-mid sized venues. In the last 10 years, they've hosted 23 events (1 Skate Canada and 1 Nationals each year, plus 1 4CC, 1 GPF and 1 Worlds) and only 5 times used a larger venue.
Especially given that their current established draws (Chan, V/M and D/R) will almost certainly be retiring after 2018, I'm not surprised they're wanting to stay in a smaller venue for 2020 Worlds.