The reasons you could not have had a MLB bubble:
Independent of the players' consent, (which they did not want one), the main argument against the bubble's viability concerns logistics. MLB's needs are so different from other leagues, in terms of size and scope, that it is thought that a complex approach would not be feasible.
There is some mathematical validity to this point. One NBA team's roster comprises 15 players; a complete MLB squad, the 30-player roster plus the alternate-site reserves, is 60. If a single MLB team equals four NBA teams, then the entire MLB would equate to about four whole NBAs (and besides that only 22 NBA teams were in the bubble). The intake process, where the players are tested upon arrival and then quarantined for a length before they're permitted to congregate and resume practice, would have required four times as many hotel rooms and beds, four times as many meals, and four times as much diligence.
"It would've been an incredibly massive undertaking," a National League executive said.
Under the three-hub proposal, MLB would've split the teams and spread the demands. The league still would have had to find a way for 10 teams to practice daily, and for their alternate-site players to remain fit. Even if the schedule was built in a way where teams split five ballparks, rotating hosting duties, those backfield scrimmages needed a place of their own.
Other complications would have included the weather, since there's only so many domed or climate-controlled stadiums to go around; the differences between big-league and minor-league facilities for training and recovery purposes; and the differences between big-league and minor-league facilities for lighting and gameplay purposes. If the
Toronto Blue Jays' forced nomadic lifestyle proves anything, it's that even the lighting is better in the Show.
It wouldn't have helped MLB's efforts to keep the season off the ground that Florida, Texas, and Arizona were all COVID-19 hotspots entering July. "The facilities exist there, obviously," a veteran American League front office member said, "but the environments are ... nope."