Because you don't just automatically win because the patent is expired. You have to hire a lawyer to research the patent and find out it's expired, then, at minimum, your lawyer has to go to court and prove that the patent is expired.
You can, in some cases, sue for damages accrued up to six years in the past. You don't have to sue for those before the patent expires. You just stop accruing new damages when the patent expires. Past damages is what was allegedly at issue here. (Which is why the attorneys were talking about "marking," as it's a factor in whether you can get pre-suit damages for some types of claims in some cases.)
