The simple answer is that it actually works for real-world software, is microarchitecturally feasible and flexible, and architecturally enforces non-forgeability (which is crucial allowing in-address-space compartmentalisation of distrusting software). Most schemes that take the metadata-table-on-the-side approach fall down on those last two points. MPX is particularly notorious for tanking performance, having race conditions (because loading the bounds is not atomic with loading the address) and having an extremely limited number of bounds registers (I think 4? which is even worse than the highly constrained register set of 32-bit x86) so you're constantly spilling/reloading bounds data from memory. I don't think any of them have been shown to work across the entire software stack from the kernel to core userspace runtime parts to graphical desktops like KDE.