It was a case of 'better safe than sorry', I think.
Although it was recognised that it was very, very unlikely the possibility that the Argentines might launch some sort of SF raid on Ascension, using C-130s (the practicalities of the C-130s getting there notwithstanding) couldn't be completely discounted. If we were fitting our Hercs with AAR probes, why couldn't they, refuelling them via the tanker-equipped Hercs in the Argentine inventory? So that needed to be guarded against, no matter how remote the possibility.
There was also the need to keep the Soviets away from the place (remember that at the time, some officials worried that the Russians might pass info to the Argentines)
I have heard a suggestion that there was a further thought that - again while highly improbable - it couldn't be guaranteed with absolute certainty that the Argentines, perhaps in a fit of desperation, might somehow manage to sail 25 de Mayo past the Task Force and Nimrod cover to get within range to fly off an Alpha Strike.
All fairly improbable, but until 2 April 1982, the government had thought that the probability of an Argentine invasion of the Falklands was fairly implausible... So ruling things out on the basis that they appeared a rather remote possibility didn't feature in the thinking at the time.