For the sake of simplicity, let's imagine this is a straightforward case where Alice alleges that Bob negligently injured her during a sporting event.
It locks you into a narrative
If Bob tells the media that, say, Alice was being careless, and that was the cause of her injury, Bob has now conceded at least the following:
- Alice was injured at all.
- Bob was present and witnessed the injury.
Additionally, if Bob wants the media to believe him, he will probably have to concede the following as well:
- Bob physically touched Alice.
- Bob was paying attention to Alice at the time.
- Some acknowledgement of how serious (or trivial) Bob believes Alice's injury to be.
If Bob is sufficiently foolish, he may also end up saying one or more of the following (all of which are basically gifts to Alice's counsel):
- Bob "made a mistake."
- Bob intentionally did [something more specific than touching Alice, such as deliberately running into her].
- Bob "shouldn't have" [done that thing].
- Bob "didn't think ahead."
- Bob "wasn't expecting" an injury to occur.
- Bob was the proximate cause of the injury.
Bob is going to be reliant on an assumption of risk and/or contributory negligence defense (or a similar argument), and will not be able to credibly raise most other defenses. This may weaken his case, both because some defenses have been forfeited, and because Alice will no longer need to prove any of the above statements. Also, Alice can depose Bob and ask him to elaborate on his statements to the press, or use his statement to impeach him during cross-examination, which is especially useful if any element of Bob's press statement is false, misleading, or inconsistent.
It can antagonize the judge
In general, judges prefer for cases to be tried in front of them, and not in the court of public opinion. This is especially likely to annoy the judge if Bob's press statement emphasizes something which the judge has decided to exclude, or facts which are not legally relevant (such as Alice's character).
Judges are supposed to be impartial, but they have to make a large number of subjective decisions throughout the trial. Not all of these decisions can be easily appealed, and due to the harmless error standard, an appeal may not be productive anyway. So antagonizing the judge is usually a bad plan.
It is irrevocable
Once you make a statement to the press, it's out there. The public will read it and make up their minds, one way or the other. Lawsuits can take months or even years to fully litigate, and what looks like a slam dunk case today may turn into a quiet settlement agreement tomorrow. It's harder to make that transition if you've released a confident press statement explaining in great detail why your side is right and the other side is wrong. This gets even worse if you actually manage to lose the case. At that point, the old press statement is just embarrassing.