I am one of the H1B people, so I can give you a little bit of insight:
Typically any H1b visa needs to be extended every 2 or 3 years. The extension process can take from 2 months to 8 months usually for approval. Nowadays due to increased scrutiny, when the extension application is pending for approval, everybody gets the idea to apply for canada due to anxiety from the wait. some of them get started by writing ielts (without knowing other details like an ECA is needed), may be they do a few more steps but usually once the extension comes and they are good for another 3 years, they forget about Canada. Only a few who felt the stress was too much or who think they may not be able to get permanent status seriously move further with the process