I don't understand why it cannot be. If a particle is "riding" a pilot wave and is guided by it, then the pilot wave must precede the particle in some way. But at what speed? Isn't the simplest explanation that the pilot wave is instantaneous, with the particle simply being a phase phenomenon of this wave? Specifically, the group velocity of the pilot wave? This group velocity is limited in some way to c, so there are no causality issues. And it seems to solve the non-locality issues of QM very nicely. I've looked at the literature a bit, including Bell's "Speakable and unspeakable in quantum mechanics," and can't seem to find any discussion of this. I'm also slugging my way through very basic quantum field theory, and see Fourier transforms being used to go back and forth between first and second quantization. Within a box, and using the boundary conditions given by this box, the Fourier summation doesn't pose a problem. But without a box, it seems that the boundary conditions could require the entire universe. Again, an instantaneous pilot wave seems to address this.