Because he pissed hot for clenbuterol at levels which strongly suggest that meat contamination was the culprit. There is precedent across sport for athletes to be found not at fault for testing positive for clenbuterol at very low levels if it can be plausibly shown that it was caused by meat contamination (it is common for clenbuterol to be given to live stock in certain countries, namely Mexico).
There is precedent in the UFC for USADA finding fighters to be not at fault when they pissed hot for clenbuterol:
https://ufc.usada.org/brandon-moreno-accepts-finding-no-fault/
This doesn't mean that this whole situation isn't problematic. It suggests, at best, that Canelo's team is unprofessional, but there is an ample amount of precedent for athletes with drastically less clout and money behind them more or less getting off after pissing hot for clenbuterol in very low amounts (which it has been confirmed that Canelo's tests showed). The fight isn't going to be cancelled (at least not for this test).