http://www.cra-arc.gc.ca/E/pub/tg/rc4110/rc4110-15e.pdf
Here's a link to the document. I think one key element that makes them a contractor is that they are free to work when they want and can work for multiple competitors. A driver can work for both lyft and uber at the same time.
They own their own tools to do the job
They take the capital risk
The worker controls profit and loss and is paid a flat rate for their services. Their efficiency at delivering the service determines profit or loss. They can't set the rate though sot hat would favour employee