40 rides per capita is pretty typical for smaller, self-contained Canadian cities. Oshawa Transit Commission had around 30 riders per capita back in the day. So St-Catharines isn't that surprising. Manufacturing towns usually have crappy ridership so I guess it is impressive in that sense.
I think places like Mississauga, Brampton, Oakville, etc. are at the most disadvantage because they are not self-contained cities, so their transit systems cannot serve all their residents needs by default. So regional systems like York and Durham are important to overcoming that problem (but on the other hand, both York and Durham have really crappy ridership...).
I think one of the main reasons Mississauga has the most ridership and service in the 905 is simply because it is the most self-contained (55% of Mississauga residents work in Mississauga, compared to 27% who work in Toronto). 46.8 ridership per capita is good considering, but it sucks compared to an entirely self-contained city like for example Winnipeg (79 riders per capita). If 100% Mississauga residents worked in Mississauga, its local transit would be even better.
Btw, I added Oakville to the list. Check it out.