In particular, they tended to roll much better than Hot Wheels.
The "axles" were some kind of fine spring steel. The matchbox cars had noticeably less drag than other brands and rolled farther and more straight. The plastic on the wheels was more flexible and smooth.
I do think that some thought went into how these things rolled. Or maybe I am mis-remembering my childhood experiences? I guess I will never find out!
As a result I inherited a lot of his toys when he grew out of them, such as his car collection (which wasn't all that huge). I bought my own ones in the early '80s and comparing them they weren't even close in quality.
The build quality of the older cars were far superior. The metal was thicker and stronger and any moving elements like the occasional opening door would last a lot longer. Sometimes you'd get a new car that could compete in a distance race down a sloped track, but in general their build quality felt very random.
There was especially one from my brother that was my favourite, I think it was a '76 Chevrolet Corvette in orange. It was extremely solid and ran very well, unfortunately at some point I tried to paint it and that damaged the axles, after which it was never the same.