Why it happens
Almost always because the landlord and the leaseholders are the same people. Where the residents have bought the freehold and hold it through a management company, collecting a ground rent means the residents invoicing themselves, paying the money into a company they own, and accounting for it. For a rent of £50 or £100 a year the exercise can cost more in administration than it raises, so at some point the demands stop going out.
That is an understandable commercial decision and it is not a scandal. It is simply not the same thing as the rent having been abolished, waived or extinguished, and the paperwork rarely records it as a decision at all — which is why it usually surfaces for the first time in a sale.
The same pattern appears for other reasons: an absent or uninterested freeholder, a landlord whose records were lost, or a managing agent change where the rent roll did not survive. The analysis is similar but the risk is not, because in those cases the people entitled to demand the rent are not the people living in the building.