Why not? I can understand because of the road behind the Kop end but the centenary? (And yes, I know about the transport requirements but, while it would take a huge investment, it could be overcome.
Road is a main trunk road behind the Kop, so highly unlikely it could be moved. Now would be the time to do it given the amount of demolition going on but seems little to no intent to do so by the club or the council. Cost could be huge too due to that, which then makes the return on any expansion, especially the Kop where it wouldn't have any corporate tickets to pay for it, unworkable.
Houses behind the Centenary are in decent condition, and as far as I'm aware pretty much all lived in. It's one thing working with the council to get land behind the Main Stand where the vast majority of the houses were empty and in poor condition - it's another to get them when they are lived in and habitable. As with the Kop, to buy them privately would cost a fortune and make any expansion unworkable. That's before you consider the work involved in expanding the Centenary which would more than likely require the Upper being removed.
The best bet is making a larger Anny Road end than is planned, but obviously there are various issues involved with that, the transport one being the largest and most costly.
A safe standing Kop could see a further 6-8k on there (roughly, could be less, could be more depending on the configuration) and hopefully lower prices - but again you'd have transport issues, and the small matter that it's currently not allowed.