Dallas was just flat-out unable to keep up with the offensive pace that San Antonio was pushing. It's not that they weren't able to run with them, just that they weren't able to make shots with them.
It seemed like every time Dallas would go on a mini-run, San Antonio would just nonchalantly knock down a few three-pointers, using one of many players. Each time Dallas tried to cut into the lead, San Antonio would take the points back, using brain over brawn, skill over athleticism and ball movement galore.
San Antonio was San Antonio. That's really the best way to explain it.
In the end, Dallas just might be a decent team once Dirk works his way back into the starting lineup, but by the time that happens, it might just be too late.