What causes some briskets to go long?

This brisket is at 20 hours and still trying to get it up to temp. I know every brisket is different but exactly what makes it take longer? Is it simply fat content?

We are in a blizzard here but my smoker's done a good job maintaining 250 degrees with a welding blanket over it.

This brisket is at 20 hours and still trying to get it up to temp. I know every brisket is different but exactly what makes it take longer? Is it simply fat content?

We are in a blizzard here but my smoker's done a good job maintaining 250 degrees with a welding blanket over it.