Why aren't they also advertising sales of tickets into Canada?

Possible rough draft memo for the advertisement:

Tickets into Mexico

We now sell tickets to your final destination in Mexico. Simply enter the city name in the destination field.
We do not trust our buses to remain in Mexico for any significant duration of time. So
Upon arriving in Mexico, you will be transferred to a Mexican bus partner who we hope will know that you've already paid us for your trip for the remainder of your trip. However, due to the recent appearance in Canada of bus riders specializing in beheadings, your ride in Mexico may be safer than across the border north of the U.S..

Greyhound - We hope that more of our passengers get there alive than on any other bus line.