Because they were 6 bill in debt and had to pay it off. I agree that selling was a big mistake since Canada was holding their losses down. The buyer did overpay for SWY though so hope they use the money wisely.
The shares of SWY are an incredible bargain at this price, given the sale of Canadian assets will bring in an amount equivalent to their entire market cap. Pay off half the debt, buy back half the shares. The shorts will be licking their wounds.