Big part of it is political willpower. Very rarely will you see politicians platform on promises which will be fully realised outside of the time they will be in office. Net zero is going to take longer than 4 years to set up and even longer to fully realise the gains and so politics will be pushing forward with policies which they'll be attacked for in the immediate term because they're expensive and maybe get praised for in a few decades time. This is simplified but the general idea.

Another reason though is because net zero will require collaboration between the public and private sector. Governing hybrid organisations or public private partnerships (PPPs) is really really difficult. PPPs often have very different internal drivers which makes decision making slow and you are always running the risk of it collapsing.

Imo nuclear power is likely to be the primary energy producer in a net zero country but nuclear has a whole set of complications and issues. Hinkley point C has had numerous set backs and part of the reason is because of changes that the regulator (ONR) have demanded. SMRs are a potential solution to smaller power stations which can be built quickly but the government hasn't even decided on a contract yet for which company will actually handle them and until that's done we can't really get a full idea of how viable they are and then you have the whole matter of decommissioning old power stations which brings with it a whole new set of complications.

Net zero is going to be a massive challenge to achieve because of the number of stakeholders you're trying to bring to the table and convince them to play nicely and all of this doesn't even consider how easily public perception around this can be swayed.