The serial connection avoids ground offset problems that seem to show up with the analog connection.

Also consider this: the wideband controller contains a cpu/mcu which drives the actual sensor and computes lambda/afr from the sensor's response... the controller then emits the lambda/afr via the serial comm port, and simultaneously maps it to analog voltage via the programmed afr:v map and emits this voltage via the analog output (only for it to be unmapped in the scantool)...

i.e. the analog connection involves extra manipulations in mapping/unmapping the afr.

The serial afr is more precise than the analog afr, especially if the analog v:afr map is too broad (e.g. 0-5V -> AFR 0-50).