Journals are typically interested in publishing novel results. Why would it be a problem if studies which find that a drug works as expected are not published, as long as they are still submitted to the FDA and assessed there?
Because it's bad both for science and for the public interest to keep this data secret. Also, many of the studies that have been suppressed do not "find that a drug works as expected".
Nobody's saying that journals have to publish every study, just that the data should be available for review. That's so bedrock a principle of science that it's hard to imagine a credible argument against it.
I think the problem is the fact that pharmaeutial companies are trying to turn a profit to ensure their survival. If you invest a billion dollars in a new drug (which is the average cost these days) and its a flop, are you going to adverise it or is it in your inerest to be silent and hope another drug company makes the same mistake as you and suffers the same opportunity costs, therefore minimising your losses.
This is a quintessential problem between public and private research.
Sounds like an incredible problem not only for bringing new drugs to market, but also for all of the medication that's currently prescribed.