Bottom indexing radical 白. Originally the lower element was a 曰 'speak' shape — many people side-by-side speaking in unison — and graphic regularization turned it into 白. Useful family clue: 皆大欢喜 (all are delighted), 人人皆知 (everyone knows).
Top 比 pictures two figures standing side by side. Here it sets the image of multiple people lined up together — what is true of one in the row is true of every one. This 'all in line' picture seeds the meaning all/every/in every case.
