Why are "canard" not used on commercial aircraft?
Well the Tu-144 was one example of an airliner which made use of canards.  The design configuration has been proposed before for large transports such as Boeing's ill-fated Sonic Cruiser.



And I suppose if you consider other, smaller aircraft, like the Paiggio P.180 Avanti for use in Part 121 and Part 135 operations, the that configuration is used often.

Probably the biggest reason is there hasn't been a pressing need to do so or the costs to abandon all of the engineering knowledge base in favor of a brand new design does not outweigh its potential benefits.