Date: December 2nd, 2019 9:36 PM
Author: I'm gay
A couple of reasons.
Until recently, there has not been a US / Canada supplier of double decker buses.
There were concerns about security on the upper level of the bus where the bus driver could not see what was going on (solved to some degree with video cameras)
The upper level of the bus is not accessible for those with disabilities, and this may affect government funding
In some cities, major infrastructure - bridges and underpasses, street trees, signs and transit garages did not allow sufficient clearance for the taller buses.
There is certainly an advantage to double decker buses since they take up far less road space per passenger. To provide extra passenger capacity on buses most North American transit systems use the extended, articulated buses which provide similar passenger capacity but take up more road space than a double decker.