AGP was released several years before PCIe specifically because the bandwidth needs of graphics cards were so high. PCI just couldn’t keep up with the demands of 3D accelerators that were starting to come into widespread use. AGP increased bandwidth massively by providing direct access to system RAM, unlike PCI which had to go through the CPU.
A better question might be why AGP didn’t supplant PCI for all devices rather than just graphics cards, and the answer is that since AGP was a port rather than a bus, it was impossible to put more than a single AGP slot on a motherboard.
Once PCIe came along and was able to provide the bandwidth and DMA required for graphics cards, it simply replaced both PCI and AGP, rendering them both obsolete.
A better question might be why AGP didn’t supplant PCI for all devices rather than just graphics cards, and the answer is that since AGP was a port rather than a bus, it was impossible to put more than a single AGP slot on a motherboard.
Once PCIe came along and was able to provide the bandwidth and DMA required for graphics cards, it simply replaced both PCI and AGP, rendering them both obsolete.