Why only one Grand Canyon

Take a look at the deep canyons that formed afterwards. Solid rock canyons were cut at St. Helens shortly after its eruption and take note of the irregular formations.

Decreasing flood water dynamics and subsequent river erosion can certainly form the formations that we see in the Grand Canyon (even sharp angled turns and horseshoe-shaped hollows). Mount St. Helens: Evidence in Support of Biblical Catastrophism

On the flip side of your solid rock wall question - why do we not see similar deep canyons formed by the world's mighty rivers?

Prior to the eruption of August 7, we drove vertical rows of 16-cm-long iron nails flush with the bedrock (breccia) wall in the stairsteps as high as 10 m above the floor. After passage of pyroclastic flows, the upper part of the stairsteps showed erosion of 4 cm or more, irrespective of height above the floor; thus, erosion was not confined to low parts of the stair steps. All nails from the stations near the base of the stair steps were removed by the flow and the configuration of the channel differed from that before the eruption. The parts of other channels low on the volcano flank also are deeper and wider with respect to the pre-May 18 stream valleys than those parts higher on the flank. Erosion was greatest where the pyroclastic flows traveled at their greatest speed near the base of the steep flank; there the flows abraded and scoured most parts of the bedrock channel to depths measured in meters. Furthermore, preliminary topographic contours suggest that pyroclastic flows of May 18 may have gouged out the base of the stair steps more than 35 m below the pre-May 18 surface. During the eruption of October 16-18, the walls of the stair steps were further modified by local removal of more than several meters of bedrock. For example, a nearly vertical, 10-m-high bedrock (breccia) cliff between the second and third step down from the top of the stairsteps became a gently sloping, 3-m-wide gully.

Coralville Lake, named for the nearby town which was in turn titled for the area's fossil coral formations (Figure 3), was subjected to the historic floods of 1993 which devastated Iowa and the adjacent midwestern states. Water began to overflow the concrete emergency spillway on 5 July 1993 and continued for a period of 28 days. A maximum estimated flow rate of 17,000 cfs was reached on 24 July following closure of the normal outlet gates for seven hours due to downstream flooding of the Iowa River (Rogers, 1993). The water level in the lake at this time was 4.5 ft higher than the top of the spillway.
Figure 4 provides a view of the resultant erosion damage below the spillway, with the concrete spillway itself visible in the background. A 15-foot channel was eroded into the underlying bedrock (Anon., 1993), exposing the Devonian limestone which in evolutionary time is said to be some 375 million years old.