It’s all the logistics and the follow-on effects.
Have to hire twice as many performers/managers/techs (can’t require seven-day work-week that exceeds 40 hours). That has the likely impact of decreasing the number of hours the current cast works. Enough hours must be available, which means three-four days for each side of casts. Doing THAT could make performing less attractive to people who like max hours of the whole event. That doesn’t account for more turnover/replacement requirements during the run.
The biggest issue, however, is house repairs/optimization. The days off are when things get repaired, adjusted, and optimized. The tech/creative team work the non-event nights to get effects right, repair damaged decor, and make significant adjustments. Those things are extremely difficult to execute in the day because (1) most of the crew works nights throughout the event (2) BTS tours prevent full access to the houses for crews to do major fixes.