This has to do with both interleaving (most SSD controllers use 8 channel with 2 die per channel interleaving; 16-way interleaving in total) and with SATA bottlenecks. Interleaving is probably best described as RAID0; a single SSD is actually a RAID0 of several flash components. The one thing it cannot improve is single queue read IOps, which remains at single channel performance (~25MB/s).
However, larger SSDs may have just as much interleaving as lower capacity SSDs, because larger chips are used instead of more interleaving. If there would be PCIe SSD controllerchips with more than 16-way interleaving, the speeds could very well continue to scale; into the multi-GB per second quite easily.