Buffalo gets the home field because they have played the same number of games as Cincinnati and have won one more game (i.e., they have a higher winning percentage). Because it was the Cincinnati-Buffalo regular season game that got cancelled, it makes it a little easier to proceed. Buffalo getting home field next week is consistent with the rules concerning cancelled games that were in place prior to Damar Hamlin collapsing on the field.
For good or for bad, the NFL altered the rules when they cancelled the Cincinnati-Buffalo with regard to the Cincinnati wild card game and the potential AFC Championship Game between Buffalo and KC. Cincinnati could have gotten screwed had they lost the week 18 regular season game and had to go to a coin flip to determine home field for the wild card game against Baltimore, but that didn't happen. KC could argue that they are getting screwed if Buffalo advances and the AFC Championship Game is played at a neutral site.