Because they lost
The procedure was espoused by Aesop in The Fox & the Grapes - don't listen to the loser when they complain that the game was rigged.
Also, don't complain that "that's not how they do it in England" - how they play they game in other places is irrelevant.
The plaintiff sets the terms of the case
Defamation is rarely about out and out lies - media organisations are too savey to straight out call someone a kiddie fondler. Instead they present information is such a way that such a conclusion might be imputed. The plaintiff in a defamation case gets to choose the imputations that they want to rely on.
In ABC v Wing:
Dr Chau alleges the publications, in their natural and ordinary meaning, conveyed the following six imputations:
Dr Chau betrayed his country, Australia, in order to serve the interests of a foreign power, China, and the Chinese Communist Party (CCP) by engaging in espionage on their behalf;
Dr Chau is a member of the CCP and of an advisory group to that party, the People’s Political Consultative Conference, and, as such, caries out the work of a secret lobbying arm of the CCP, the United Front Work Department (United Front);
Dr Chau donated enormous sums of money to Australian political parties as bribes intended to influence politicians to make decisions to advance the interests of the Republic of China, the Chinese government and the CCP;
Dr Chau paid Sheri Yan, whom he knew to be a corrupt espionage agent of the Chinese government, in order to assist him in infiltrating the Australian government on behalf of the CCP;
Dr Chau paid a $200,000 bribe to the President of the General Assembly of the United Nations, John Ashe;
Dr Chau was knowingly involved in a corrupt scheme to bribe the President of the General Assembly of the United Nations.
Dr Chau further alleges the conveyance of a true innuendo to the effect that he broke his pledge of loyalty to Australia as an Australian citizen.
In their defence, the respondents alleged that each of the above imputations was substantially true. Further, the respondents sought to allege substantial truth in respect of variants of the applicant’s imputations. Each variant imputation corresponded to those set out above but was prefaced by the words ‘There are reasonable grounds to believe that…’.
The trial judge, Rares J, rejected that defence and refused leave to file an amended defence because "there was a substantive distinction between suspicion and fact, rendering each of the respondents’ variant imputations impermissible variants of Chau’s pleaded imputations. His Honour also determined that the proposed particulars of truth in the schedule to the defence and the proposed amended defence did not provide a basis for the inferences alleged to arise therefrom, did not properly particularise allegations of knowledge, failed to comply with the rules of pleading, and could not reasonably support the plea of truth to the applicant’s imputations." All of the trial judge's findings were upheld on appeal at the Full Bech of the Federal Court and the High Court did not give leave to appeal so this is the definitive law in Australia.
That is, the imputation that someone is suspected of doing A is different from the one where they did A. Obviously, a publication can convey either or both of those imputations but if they do contain both, the plaintiff gets to decide which one they want to sue over.
In their Honours’ view, only a precise analysis of the words used and their context will reveal:
…whether a matter is capable of conveying imputations of both suspicion and guilt, or grades of meaning in-between, including whether the suspicion imputation is one of strong suspicion, reasonable suspicion or mere suspicion, or whether it is objective or subjective, active or passive… .
So, the preliminary ruling was that they couldn't run a defence that it was true that Dr Chau was suspected of the things they said because his suit was based on the imputation that he did those things.
In the immediate sense, the effect of the Full Court’s decision has been to deny the respondents a defence of justification, without which they are left to argue that the imputations pleaded by Wing are not conveyed by the publications, and otherwise rely on a defence of qualified privilege.
That is, as they couldn't raise the truth of the suspicion, they had to demonstrate that they did not impute that Dr Chau did these things. And they couldn't. So they lost.
As an aside: these allegations and documents supporting some of them have been raised in Parliament both before and after the trial. Parliamentary statements are privileged which means that the parliamentarian who made them cannot be sued for defamation.