The fact that fuel delivery has greatly improved and that fuel dilution upon cold start could be much worse in carburetted car than with port fuel injection has already been mentioned. But there are also two other factors.
In the 1980s, there was a sludge epidemic (probably due to not-very-good VII), which lead some manufacturers to order shorter OCIs. Then, most modern engines keep oil temperatures much better under control. Take my 1985 Saab, for example. Although equipped with an oil cooler, you can easily get oil temperature in the sump to more than 130°C (turbocharger not water-cooled). Do that few times to a mineral oil with cheap VII, and it's toast. One spirited drive home for the weekend (600km in 4 hours or so, which means that to make up for all the construction sites and speed limited areas, you have to really drive flat out on any unlimited stretch), and a fresh semi-synthetic will be pitch black and idle oil pressure drops and you will have burned nearly a litre of oil in one single night. A fully synthetic, subjected to the same treatment, still looks new (and you will just loose 0.25 litres on such a drive).
Or take a buddy's Saab 99. 2-litre naturally aspirated, a single carb, only 100hp. But no oil cooler. Also relatively short geared, so high rpm in 5th gear. Having fun on a trackday or on a serpentine road, or just commuting on the Autobahn, this engine sees the same temperatures as the abovementioned turbo under full boost.
On the other hand, a new Golf VI GTI will keep the oil around 100°C, no matter what. Only once have I seen oil temperatures of 125°C, and that guy drove like a madman, digital throttle and always to 6000rpm before shifting. (Driving was slightly on these kinds ofroads, but quite fun and fast.) But even then, oil temperature peaks not exceeding 125°C at basically full load, rev limiter is a different story to 135°C when just cruising down the Autobahn at a mere 150-160km/h with the Saab 99...
So, it's basically two reasons:
1) average oil quality has improved significantly
2) modern engines are easier on the oil (port fuel injection and better temperature management).
But even one of these would be enough to greatly extend OCI. I have two Saab 900, one 900 turbo 16 from 1983 (Bosch LH jet electronic fuel injection), which was my daily driver until last autumn, and a 1983 900 GLE automatic (Bosh K-jet mechanical fuel injection and a 3-speed automatic, which means absurdly high rpm). Manufacturer recommend OCI is 7500km for the turbo, 10000km for the GLE (severe service cuts that in half).
With my driving style, that is mostly long-distance commuting at moderate speeds and a few drives for drivings sake, and perhaps one full-throttle 600km-run over an OCI, at 5000km any semi-synthtic 10w-40 were toast. Pitch black, but more importantly the 16V turbo would develop lifter tick when hot, the 8V automatic piston slap. That tells me oil has thinned out considerably. (Unfortunately, I never did an UOA back then, so I can oly judge the oil indirectly.)
With synthetic 0w-40, and driven the same way, after 10000km the oil looks new and
UOAs show that it was indeed nearly virgin and I could have run the oil MUCH longer. That is still the very same car from the early 80s that killed a semi-syn in sometimes less than half the distance.