RMT (the union) mainly (there may be actual technical reasons too for some lines, but any technical reasons are likely to largely be a result of knowing that there's no point in investing too aggressively in enabling full automation if they won't be able to make full use of it). Notice the page takes care to point out that current drivers jobs are safe.

It's likely that London Underground is basically betting it'll be easier to just hire at less than the replacement rate and gradually automate more and more rather than risk massive conflict with the RMT for years.