Demographics I imagine will play a part. Declining fertility rates and people living longer means more demand on the Health Service and spending has not matched that demand. Secondary, rather than primary care is likely to be the biggest drain with shifting demographics.
Then there is pay and conditions, training, staffing all impacting on the service provided.
Past spending as a share of GDP may have been sufficient for the demand at the time, but like other services - education, defence, policing - there has been underinvestment over the last decade or so. Which, given we kept electing Govts obsessed with austerity is hardly a surprise.