Also, there many craters before and after Chicxulub was created, but the KT boundary is the only one commonly mentioned…why?

Because it's the first one found, a nice story of serendipity, because it's the last really big one, and the one that killed the dinosaurs. Everything you need to make a good story - even though it's not unique.

Iridium (and other PGE) anomalies and pretty much common feature of impacts. This paper shows attempts to link mass extinction events with impacts. This is a controversial issue, because you can also link massive volcanic events with mass extinctions, not only impact. But that's not for now - they do show iridium anomalies for all impacts. That's another paper showing iridium anomalies for impacts, much older this time.