There's a limited amount of equity (outstanding shares) available so to short you need to borrow the shares & pay the fee. With futures, new contracts are "created/destroyed" every day so if your order can't be matched with another trader/hedger then the order will create a new futures contract. Creating a new contract removes the need to borrow so no fee.
The hope of huge gains. The put calendars do not offer the same multiples on amount risked. (When I tried, it was bear call verticals - got assigned more than once, and discontinued)
Based upon the info the OP gave I calculated his approximate position. "The loan rate was approximately 1%/day. The 15k in stock loan fees suggests very roughly you were short maybe 72 options with stock value at roughly 1.5 million."