It's just something that's become associative with SoulCalibur, much the same as its robust customization mode. SoulCalibur II was arguably only as popular as it was due to the guests, particularly Link and Heihachi. Spawn was the weakest link, though he still had fans for sure. People lamented the fact that SoulCalibur III didn't have a true guest (only Namco cameos in custom parts: KOS-MOS, Gil, Valkyrie, Neneko, Jin, and King), and so they made a return in SoulCalibur IV, for a cross-promotion with The Force Unleashed, bringing us the Star Wars folks. Broken Destiny picked up Kratos, of course, due to being Sony-only, because it could. Then SoulCalibur V got Ezio, who was very popular and fitting the setting, unlike some of the ones that came before.
We really just have to live with it, at this point. Just be happy that Necrid will likely never return.