This article sums up the thinking in the decision:
"The absurdly divergent, often negligent approach to COVID-19 in the United States — where containment efforts are an abject failure rooted in stubborn, intentional ignorance — stands in sharp contrast to the calculated caution employed by the federal government here.
The Blue Jays’ schedule includes three trips to the outbreak epicenter of Florida, including next week to open the season in St. Petersburg against the Tampa Bay Rays, as well as one to the hotspot of Georgia for a series against the Atlanta Braves. The Rays and Miami Marlins each make one visit to Toronto, while the other teams they’d be hosting will have also visited places where cases are spiking.
So even with Major League Baseball’s protocol featuring tests every other day, even with an additional test for everyone in Toronto – nasal swab instead of the less intrusive saliva to increase accuracy – the frequent border crossings were an impossible risk variable to resolve.
Hence, the travel element avoided by the NHL and NBA with their bubble-protocols — but that MLB chose to forge ahead with — ultimately sunk the Blue Jays’ endeavour. Marco Mendicino, the Minister of Immigration, Refugees, and Citizenship, said in a statement Saturday that “based on the best-available public health advice, we have concluded the cross-border travel required for MLB regular season play would not adequately protect Canadians’ health and safety.”
“Of particular concern,” he added, is that “the Toronto Blue Jays would be required to play in locations where the risk of virus transmission remains high.”
In saying as much, Mendicino not only highlighted the dissonant paths taken by Canada and the United States in regards to the coronavirus, but also implied that the MLB plan isn’t foolproof enough to avoid the players acting as vectors for the disease."