Silent and Sound Mixed?

This movie starts out as a silent, then has sound effects, and finally dialogue with sound. Then there are parts that are silent, mixed with parts that have sound. I know "talkies" were new in 1929, and there were silents still released even later, but I was wondering why this movie switched back and forth? Was it simply budget, or did they add the talking scenes to improve box office receipts? Could it have been a stylistic tool used by Capra?

Nobody gets to be a cowboy forever.