according to the commenters on reddit, the rationale behind ending it early is that this is how clinical trials work. A trial is run until it is statistically clear that a drug is either better or worse than the current market leader it is being compared against. The trial might be ending earlier than expected, but it is not being cut short. The success metrics were reached.
Clinical trials are monitors by IRBs (institutional review boards) that independently monitor the trial to ensure that it is being conducted ethically.
It's not odd at all for a trial to be ended early if the drug being tested is obviously better than the control arm. It's especially true in trials that are testing drugs for potentially lethal diseases like prostate cancer.
What likely happened was that an interim data review was conducted, the results showed that the new drug was substantially better than the control arm (and statistically significant) and the IRB and sponsoring company decided that it had an ethical obligation to end the trial and provide the drug to all the patients.
