Nikon had a compelling reason to offer it. Their pro series DSLR's were crop cameras, and so were the lenses. Photographers with a ton of money invested in pro grade lenses wanted to be able to use them when Nikon went to FF. The crop mode is offered for those who bought those expensive DX lenses.
Canon did not sell DX lenses for their Pro series cameras, just FF lenses. When the Rebel Series came out, Canon offered EF-S lenses as a low cost option for a Standard Zoom and wide zoom. They will not physically fir on a FF body. So why would Canon crop a image for lenses that would not fit??
If you just want a cropped image, its a trivial thing, it can be done automatically as you import into photoshop, and, if you decide to change the crop a little, you can.
Except that the UWA EF-S lenses outperform similarly priced UWA FF lenses in the crop area. For those who have invested in a crop body with a couple of lenses (especially hobbyists) allowing FF bodies to take EF-S lenses gives them options. Unless Canon thinks that the users are too naiive to see the EF-S Vignette on FF sensor and complain about it.
But, they Do not fit on a FF, they hit the mirror and cannot be mounted for that purpose. Why have a crop mode for lenses that cannot be mounted?
As far as 3rd party lenses that will mount, I do not see Canon doing anything to support or encourage buyers of the competition.