I think they latched onto that because it was completely in violation of the 2002 McCain-Feingold Campaign Finance Reform Act:

SEC. 313. USE OF CONTRIBUTED AMOUNTS FOR CERTAIN PURPOSES.
(a) PERMITTED USES-

...
(b) PROHIBITED USE-

(1) IN GENERAL- A contribution or donation described in subsection (a) shall not be converted by any person to personal use.

(2) CONVERSION- For the purposes of paragraph (1), a contribution or donation shall be considered to be converted to personal use if the contribution or amount is used to fulfill any commitment, obligation, or expense of a person that would exist irrespective of the candidate's election campaign or individual's duties as a holder of Federal office, including--

(A) a home mortgage, rent, or utility payment;
(B) a clothing purchase;
(C) a noncampaign-related automobile expense;
(D) a country club membership;
(E) a vacation or other noncampaign-related trip;
(F) a household food item;
...


I think in the context that it flew in the face of everything McCain was supposed to stand for, it was a very important indication of her character.