Only business return
I book 2 separate 1 ways, so I didn't get your question.
I see what you mean, there are only bus seats for Sat, sun , and mon or 4/20 (sun) tues there's economy availability, but I have no thoughts as to why the other days don't even have full bus availability.
I just checked the dates. You're trying to fly back on sun, day 4 of chol hamoed.
It's early enough to get paying flights at a decent price. Remember the Canada TO USA leg has $50 in taxes on top of any points price, so a $160 cash ticket is actually not that much more than an Avios ticket.