2 good reasons:
- its VERY long
- its either expensive (sleeper cabin) or tiring (in coach & sleeping in your seats, and still more than a plane ticket)
I think it would help if wed understand why you want to do that. Do you only want to do it one way to see the country? If so, then it can be nice. The train is normally timed so you can see the nice scenery (ex. the Rockies) and sleep while it is more boring (like the North of Ontario). It can however be late like many other transportation methods, and as it shares the rail, so there is always a possibility of missing that.
If you decide to do it (whatever your reasons) in coach seats, it might be best to rest a full day in Vancouver before the cruise, because all my friends who did that were quite tired at the end. Other people I know did it in a sleeper cabin, but they worked for the railway company so paid very little. They liked it, but it still take almost 4 days.
Hope this helps!