I think there were two main causes: cell motion nearly parallel to convergence boundary (front) and an imbalance between capping and forcing...
First off, model forecasts called for storm motions to the east or perhaps east-southeast. The storms in the initial cluster near Dickens, TX, moved northeastward for the most part (some ENE). So, given an area of sustained convergence, storms would move off to the NE, or ENE, only to have more storms develop to their southwest. The orientation of the storms meant that the precipitation from one storm fell into the inflow/updraft of the storm to its northeast.
Second, ample sunshine in the area meant that the convective temperature was likely reached over a relatively large area. Without any real cap, the strong convergence along the front and various OFBs (or along each particular storm's front-flank outflow).
IMO, upscale growth from supercell cluster to MCS occurred relatively rapidly mainly because of the two above-noted factors. Now, another question is why the strong low-level shear didn't yield a strong supercell, around which there should have been compensating subsidence. Since we're all chasers, we know it certainly isn't uncommon for initiation to yield a cluster of storms, with one or two updrafts dominating in time. Heck, this happened 5-29-04, among many other days. On those days, however, the shear vector was more normal to the convergence boundary (dryline on 5-29-04).
Looking at 0z data and model initializations last night, I almost cried. Assuming the model initializations were at least close to correct, the area along and immediately north of the stationary front was moderately unstable and moderately/highly sheared. Re really don't see the juxtaposition of strong shear (250-500 0-3km SRH, 50-90kts effective shear, etc) with strong instability (2500-3500j/kg SBCAPE) very often. As I noted in my REPORT post, it appears that prior model runs underforecast 850mb flow... Well, I guess it goes to keep all of us on our toes...
Biggest lesson learned: It doesn't really how much shear / instability is present if the storm mode is quasi-linear instead of discrete supercells. I strongly believe that, had we had discrete supercells near or north of the Red River yesterday, we would have seen tornadoes, and I still think there was the definite possibility of a strong tornado owing to the strong low-level shear. Instead, we saw a relatively rapid transition to MCS. There was a supercell that developed near or just norht of CDS that had decent rotation, but that was before the MCS-like convection south of Quanah moved northeastward and intercepted the warm, unstable inflow.
EDIT: I agree with Mike. Some of the biggest tornado days have seen many supercells in a relatively small area. As he noted, 5-3-99 had several supercells in very close proximity that still produced long-lived, significant tornadoes. I think this comes down to the orientation/position of the supercells relative to each other. Instead of one supercell precipitating into another's updraft/inflow (such as was the case yesterday I believe), the supercells on 5-3-99 were able to remain in close proximity, yet their position relative to each other didn't allow for "negative" storm interaction. I think such storm-scale things are largely unforecastable given what we currently know and can measure/forecast...