To a fairly significant extent, the Morris worm is why there haven't been more; it did prompt something of a culture shift away from trusting users to trusting mechanisms, mostly by prompting people to realise that the internet wasn't only going to be in the hands of a set of people who were one or two degrees of separation apart. It didn't make sense to assume people would treat it with reverence like a giant beautiful shared space.

It's most obviously paralleled by Samy Kamkar's MySpace worm, which exploited fairly similar too-much-trust territory.

I imagine the - heterogeneity of modern computing environments - number of 'layers' in any system - sheer size of the modern Internet all also make it harder to scale