Many of the modern exhaust manifold issues have started with failing fasteners. Past that it may be related to the extra weight of supporting the cats, and then dealing with the higher exhaust temps because of the cats. I'm sure the bean counters "cost reduction" and the inexperienced modern educated engineers that design the exhaust systems as a whole also plays into the manifold problems. When this stuff was designed, it was all supposed to be replaced and junked in 15 years or 150K miles.

The modern exhaust manifolds also live in a much different environment then those of 30 years ago.