Other restrictions besides bandwidth exist. For example, DPI requires more CPU and memory, which are some of the major constraints of networking equipment, especially when the equipment resides at the edge (borders) of a Tier 1 ISP.
from (http://en.wikipedia.org/wiki/Narus_(company)) we have the following "A single NarusInsight machine can monitor traffic equal to the maximum capacity (10 Gbit/s) of around 39,000 DSL lines or 195,000 telephone modems. But, in practical terms, since individual internet connections are not continually filled to capacity, the 10 Gbit/s capacity of one NarusInsight installation enables it to monitor the combined traffic of several million broadband users."
