The facetious answer would be 'because the summers were always better back then', but more seriously, it could because there are more varieties of more virulent blight around nowadays. Importing plants and seeds from all over the world has tended to accelerate this trend.
As we all know, there are few years now when you can grow potatoes or tomatoes without them going down with late or early blight. Even here in France where it tends to be warmer and drier, I have been told that most of the local polytunnel tomatoes have had to be destroyed due to blight.
So far, touch wood, although I use an overhead sprinkler system, our polytunnel tomatoes are surviving though lots of lower leaves have gone brown and look as if blighted. The stems are ok so far.
Anything that spread the fungal spores therefore, such as water droplets in a sprinkler, should be avoided, and drip irrigation is probably the best policy in blight affected areas.

Blight is not transmitted by water. It is predominantly an air borne fungus that needs moisture on plant leaves for it to take hold and spread.
So watering at the stem is not causing the blight. Rather, humidity in the greenhouse. For it to get into the greenhouse in the first place means it has blown in from infected potatoes or tomatoes outside somewhere.