It's an Olympics thing, same thing in track and field, max 3/country to have a broader field and more countries involved. This used to be most significantly obvious in the steeplechase where there were years where 15 of the top 16 in the world were Kenyan, but only 3 could go to the Olympics. This is why in many cases, you have stronger fields in Diamond League or WTCS races, because the caps per nation are higher...
In general, for most countries, there's a churn with top Jr./U23 athletes getting chewed up, and few realizing the same success at the top level. Canada has had a number of promising athletes come through the Jr/U23 ranks, and then have very short stints at the top level, often due to injuries. I think in general there's great coaching in Canada, but the challenge tends to be access to funding to continue. So often there's a lot of pressure to perform in that first year or two in the Sr ranks in order to show podium potential in order to gain access to support, otherwise they tend to move to long course or leave the sport all together.
In general, for most countries, there's a churn with top Jr./U23 athletes getting chewed up, and few realizing the same success at the top level. Canada has had a number of promising athletes come through the Jr/U23 ranks, and then have very short stints at the top level, often due to injuries. I think in general there's great coaching in Canada, but the challenge tends to be access to funding to continue. So often there's a lot of pressure to perform in that first year or two in the Sr ranks in order to show podium potential in order to gain access to support, otherwise they tend to move to long course or leave the sport all together.