Dementia is not a single disease, and it cannot be 'cured' by a magic drug, therapy or treatment. Dementia is essentially a loss of function in the white matter in the brain and can have multiple causes as well as multiple severities, aetiologies and pathophysiologies.
The primary cause of dementia is getting old! Your body (biologically) is designed to live until reproductive age, pass on your genes to the next generation and then die. This has worked well as a strategy for millions of years and then we tried to prolong our lives with the advent of medicine and healthcare. We didn't do very well at first - blood letting was never going to be a long term solution! - but with more knowledge and advances in medicine, most notably antibiotics, as well as better diet and nutrition, we have succeeded in prolonging life considerably. The downside to that is bits of the body are now wearing out before the body as a whole dies, so we are reaping the consequences of that in an aging population. Dementia is one example of this, but also the rise in cancer prevalence and other disease of 'old age' such as stroke (old age in this context being defined as past reproductive age).
So the prevalence of dementia is increasing because the population is increasing. Since dementia is due primarily to the brain 'wearing out' there is not a clinical treatment that can be applied to 'cure' the condition. Sure, we can delay its progression (possibly) and treat some of the observable phenotypic changes such as amyloid deposits, but we cannot 'prevent' dementia. Since it is not a clinical disease with a defined cause, and produces a range of diffuse symptoms such as memory loss, it can also be difficult to diagnose. It requires multiple memory tests, cognitive function tests and ideally an MRI to study the brain. These are time consuming (cognitive tests) and expensive (MRI). The treatment of dementia is really management of the condition rather than 'curing' the disease. This requires round the clock care in extremis, and is phenomenally resource and labour intensive.
So we have an almost perfect storm of a condition which cannot be diagnosed with any great deal of accuracy, which we know little about, which has multiple aetiologies and pathologies as well as progression rates and consequences, and is expensive to treat and manage at a time of great pressure on both the national budget and the healthcare budget. Currently we are not catching dementia early enough, which means its progression cannot be delayed by current therapies. Increasing the diagnosis rates by better screening and catching the condition (to the best of our ability) earlier allows progression to be slowed and the more severe (and expensive) management options such as nursing homes to be delayed or avoided. Incentivising GPs to 'look out for' dementia, or to refer for cognitive testing at risk or suspected cases is one way to increase detection rates. It is controversial since GPs stand to profit personally, but other options such as blanket screening the population at the age of 60 (insert age of choice) are more expensive.
Although conspiracy theorists would allege plots by GPs, drug companies or nursing home supervisors, it is simply the system reacting (belatedly) to an increasingly prevalent healthcare condition which is on the rise. Next year it will be another condition (eg see Ed Millibands recent announcement on cancer waiting times), a few years ago it was something else (4 hr waits in A&E for example). There is no conspiracy, just an aging population!