I have a really high standard for my blog posts. They go through several rounds of rewrites, with feedback from friends, before I'm happy with them. That plus the length (median ~2000 words) means that most of my blog posts take weeks or months to write. I can hammer out a tweetstorm in 20 minutes.

(Also, tweets are a fun format! I want each tweet to be a complete idea, which is hard when you have only 280 characters.)