Because there's no intentionality involved, and it's unlikely to escalate into a military conflict. "Ukraine fires an S-300 at a Russian missile but it misses

Because there's no intentionality involved, and it's unlikely to escalate into a military conflict. "Ukraine fires an S-300 at a Russian missile but it misses and kills a couple people in Poland" is a tragic accident, probably newsworthy, but not in the same way as Russia or Ukraine deliberately launching missiles into Poland would. I think Russia deserves most of the blame for this incident, since it wouldn't have happened if they weren't trying to destroy their neighbor. Zelensky continuing to insist the missile that hit Poland was launched by Russia would be news I suppose. I think that's what he was saying early on, but I haven't been following the story; maybe he gave up on that once more information came out.