Conventional PCI can also do in-band interrupts (the mechanism is exactly same), but many devices do not support that.
The reason is that PCI is multi-master by design and PCIe uses duplex packet-switched serial links. There is no master who decides who gets to use the bus at given moment (for parallel PCI there is, but it is relatively simple circuit that is conceptually part of the bus itself, not of particular device connected to it).
The reason is that PCI is multi-master by design and PCIe uses duplex packet-switched serial links. There is no master who decides who gets to use the bus at given moment (for parallel PCI there is, but it is relatively simple circuit that is conceptually part of the bus itself, not of particular device connected to it).