Because Tetenal wants to do business in the US. It's that simple. But we have multiple suppliers for RA4 chemistry.
The issue, is that several chemicals in the process are considered corrosive enough to be classed as dangerous goods. B&H can get Tetenal, because Tetenal is willing to do the extensive paperwork, and underwrite the cost of international DG shipping, for a large customer like B&H.