The pilot not taxiing often has more to do than simply monitor the taxi route & check the charts. In our outfit he/she does the take-off review & the before take-off checklist.
The take-off review involves a run around the cockpit calling a number of settings & and visually checking them. If you don't want to break the flow & miss something, you will have your head inside the cockpit for a while while you are doing this, followed by selecting the before take-off checklist. I'm not saying that it is right, it is just what my outfit expects us to do.
The taxi in this case was quite short & it is possible that the captain had his head inside at the critical moment that the aircraft was turned onto 'B'. Not the best time to be 'heads down', but you could be forgiven for thinking that it was a simple taxi route & not prone to error. Particularly if you hadn't been there before.
So it is not necessarily the case that the captain had more capacity to spot the error. He may in fact have had less!