The reason for this combination would be that when Protoss has already reached such a strong unit combination, regular low tech and mid tech units like Zerglings, Hydralisks, Mutalisks and even Guardians all simply melt without causing significant damage due to low health and the fact that these small units clump up making both psi storm and Scarab splash very effective in getting them all slaughtered.
One strength this unit combination has is the low amount of them so they should be easier to micro in that sense. With Ultralisks, you have units that have very high health, armor and speed, which means that psionic storms are no longer as effective, and even though scarabs are more effective vs Ultras than psi storm, even their effectiveness is slightly reduced since their splash is less useful. The Ultralisks merely have to stay alive long enough for the Lurkers to cause enough damage to the main targets. The main targets being the Reavers, since they are essentially a perfect anti-Zerg-ground-unit counter. If the Lurkers have enough time to destroy them, then it doesn't matter as much if psionic storms, Dragoon shots, or even Reaver Scarabs destroy them, since after the Reavers are gone, Zerg can concentrate on more cost effective, mobile and fast to produce units like Zerglings and Hydralisks, potentially with Ultralisk support as well.
The obvious problem is the cost of these units. Of course this unit composition would only be used in the late game in specific scenarios, so perhaps that gives the Zerg time to prepare to save up Vespene gas. If that's still not enough, perhaps the Zerg can try to buy time by placing lots of Sunken Colonies at bases, unless of course the Reavers are going to attack with the Protoss army already, in which case Zerg might have to sacrifice a base. It could be that Zerg needs more expansions earlier with Sunken Colonies to defend. Perhaps this is only possible with either bigger maps or with maps, where the expansions that are taken much later have extra Vespene geysers.