Apart from the fact that the necessary web APIs have only recently started to materialize, it's taken this long because it's taken this long for developers to "

Apart from the fact that the necessary web APIs have only recently started to materialize, it's taken this long because it's taken this long for developers to "get it". Programming multi-user software that spans the network, i.e. is partition-tolerant, is different to the last few decades of single-user, single-node software we've been writing. It's a new paradigm. We've known about the fallacies of distributed computing for some time. But as developers we're taking our time coming to understand them. For example, there are very few web apps today that can manage the synchronization/consistency/migration/offline access/memory/authorization/versioning/database implications of say 4GB of text data per user on multiple devices, whilst facilitating sharing between hundreds of user accounts. It's Distributed Systems 101. Up until now, these ideas have been tried out in privately managed data-centers. Now they must be rolled out to the web and the code must run in different browsers on devices of varying capability. The frameworks for doing that do not yet exist. They're being built.