There's already a thread near the top of page one for discussion of Grey's, so I'm locking this as a duplicate. There are so many shows discussed in this forum that we want to make it easier for fans of all the shows to find open discussions of their shows.But to answer your question...in the writers' blog, they said it was deliberate:
It was a conscious choice on my part to not show Burke until the end except for the flashback at the mixer. Because I wanted you to watch the episode and either a) pine for his presence or b) forget about him – until Cristina walks into his room at the end of the day. Then I wanted the power of what he means to Cristina to overwhelm us when he asks her how she is and then she begins to cry.
http://www.greyswriters.com/2006/09/shonda_on_the_s.html