BY the end of first season of Breaking Bad, we see that Walter and Jesse steal a barrel of methaphamine from a secured warehouse. At this point, Hank knows that there is some new player in town dealing in meth. But a figure named Heisenberg or Blue Meth as a product is not established. Thus, Hank stil doesn't have the specific Heisenberg case under him.
A robbery at warehouse should be concerned case for local APD and not DEA until and unless some actual drug is stolen in mass. So why are Hank and Gomez specifically sent with the robbery clip of an chemical? Yeah, it is a major ingredient of Meth, but the point is chemical was stolen and not a finished product of a drug.