Knowing something about high voltage gas and vacuum switches and ceramic insulators having worked on them for years I think it's down to the ceramic insulator material used today. Now taking that knowledge and looking at a spark plug I have a theory about what's happening but I no longer have access to stuff like electron microscopes to look for contamination of a cross-section of the ceramic surface etc.
Basically spark plug manufacturers today make plug down to a cost and so have a manufacturing process that produces plugs that suit modern computer-controlled injection electronic ignition where they can start an engine cold without using a rich mixture and so reduce the emissions. The upshot of this is the spark plug never sees anything like the rich mixture seen in our old engines and so the spark plug manufacturer can use a lower cost ceramic which I suspect has a coarser grain structure. I don't know if spark plugs 50 years ago had glazed ceramic insulators but looking at new plugs today they don't appear to be glazed. Now these modern plugs appear to fowl easier with a rich mixture and once that occurs the plug is scrap and this is what we are seeing with modern plugs in our old engines fed via carbs. I have taken such a fouled spark plug and tried to even bead blast them clean and they still won't work so the contamination has penetrated the grain structure of the ceramic surface and is not simply sat on the top of the surface.
Please note this is all just a theory, but after suffering problems with Champion plugs in my Vincent, but not in modern cars, I have found Denso Spark plugs to be one of the better ones for older designed engines and have used in my Vincent the Denso W20EP-U for the last 15 years and the last one I changed was as 20,000 miles and still working fine. I have stocked up with enough so I will never have to buy a spark plug for one of my bikes ever again, but beware and always buy from a known source as yes there are fake plugs out there, probably made in China.
Just realised I probably started a panic buy on Denso plugs, but at least it different from panic buying toilet rolls"
Simon