Put simply, the answer is because trending favors more recent posts. Basically, each topic is assigned a score, based on when it was posted, replies, and likes (it is more complicated than that, but that's a pretty good approximation of what it does). As a post gets older, it requires more replies and likes to have the same score, to the point that it quickly becomes impossible to keep up with new posts. Combined with the age out mechanism that will quickly "disqualify" a thread for trending after a certain cutoff, it can have the effect of magnifying this date-focus. I'm working to improve this so that this effect isn't so pronounced, but it is slow going (doing anything that affects the performance of the forums is slow, there are millions of posts, and it is incredibly easy to make a change that appears good but breaks everything else).