I don't think the kernel complexity and maintenance burden was deemed worth the effort. But now 1) Spectre has made syscalls more expensive and 2) Linux has destroyed all the competition and so there's alot more pressure to add features for what effectively would only ever be used by niche use cases--you don't add io_uring to benefit the Node.js crowd, you add it for Amazon and Netflix and other consumers that have have no other avenues for optimization.

To be fair, io_uring seems to be a pretty sane design. But, for example, for many years anything with a shared ring buffer between userspace and the kernel was suspect. Now there are several such interfaces. Overall tolerance has grown, especially on the part of Linus. The kernel is much larger, and Linus seems to have become much more deferential to subsystem maintainers as his ability to understand and opine on code has slipped beyond his grasp.