Why does 1080i/720p standard input require two ADV2x2s?

The data-input rate limits as specified in the datasheet refer to active resolution. Each ADV2x2 can handle a maximum of 65 Msamples.

1080i active resolution = 1920 x 1080 /2 x 60 = 62.208 Msamples/field/component. For YCbCr 4:2:2 input, this number is doubled, and 124.416 > 65, so two ADV2x2s are required.