Because the flashbacks run the risk of being repetitive if they slowly go from one event to the next. I like the flashbacks but over the course a single season not much has changed.
To make things interesting they should have some jumps ahead. Perhaps even the flashbacks will end this season with a cliffhanger. With next year picking up some time later.
I am going to be fascinated to hear how far in advance they plot the flashbacks and how they shot them. I think in some episodes those are there just because its part of the format. Not because it adds much to his back story. If the flashbacks were a edited together like a movie some of this stuff would be edited out for pace.