I suspect it's more the cumulative effect of low income and small reserves, due to the market capture by big supermarkets/suppliers, the chaos of the subsidy arrangements post Brexit, plus the costs(time and money) of ensuring you fit in with the IHT rules.
Whilst it's true most small farms should evade the tax, they can't evade the paperwork, under which they are close to drowning anyway.
The big conglomerates with an army of consultants to sort it all out, escape.