[eno_]

A FedGov lawyer would tell you, with a straight face:

Because the money in the banks crosses borders.

Because the suspect might have recently crossed a border.

Because the jurisdiction here is well inside the lines by precedent (i.e. other outrages are even worse so leave us alone about this one).

[GovernmentShrinker]

Among other technicalities, because the deposits in the bank are insured by the federal government (via FDIC).