You raise a very important point. Many people think immigrants who come to Canada automatically stay. The truth is, as you said, a very high percentage leave.
I think Jeffrey has the right idea: support certainly matters. I think there's a bit more to it than that as well. Obviously, in a newspaper article you don't get all the details. The rate at which people leave, especially in the economic class, moves with the business cycle. Economic class immigrants who arrive in a boom are very likely to stay. Economic class immigrants who arrive during a recession are much less likely to stay. Partly that has to do with a welcoming group, but partly it has to do with their success in the labour market and their opportunity costs, their potential success back home or in some third labour market. Family class individuals are much more likely to stay.
It used to be, I'll call it the old days, before 1990, that we had a system where the economic class and the family class both moved with the business cycle. During economic booms, the percentage who were in the family class would go down, and during economic busts, the percentage in the family class would go up.