Primarily because RAF has zero helicopter AAR tankers. The drogue/basket for helicopters is quite different than the one for fixed wing aircraft and RAF has none. What tanker capability RAF has is dedicated to fixed wing AAR. And I'm not certain but I think all of RAF's tankers are jets, so they can't go slow enough to refuel a helo anyway. RAF's A400 could go slow enough to do the job, but the A400s delivered to RAF do not include wing refuel pods. The UK Parliament appears to be loathe to fund any kind of helo AAR capability.
Interestingly, US Coast Guard (which belongs to Homeland Security and not DoD) has C-130 tankers and AAR capable SH-60s for long range SAR duties. So it's not necessarily a military-only requirement/need.