Maybe the reason for GCSE(whatever that is)'s choice is that AXAXA can be misread. It looks as though lines 2 and 4 rhyme with each other. Of course, the Xs would be Bs if they rhymed, but a reader may not know that. ABACA cannot be misread that way.
It might, by more knowing readers, be misread as suggesting that the B and C lines will be rhymed later in the poem, but it's standard, when only one stanza is patterned, to assume that the pattern starts fresh each time. (If there were only unrhymed line, ABAA say, one might assume it continued BCBB, but with a B and C both to deal with there are two many possible assumptions to make any one assumption viable, it seems to me.)
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