Stress
As an investigator, you'll be responding to issues immediately after they happen. Strangely, fires don't really wait for you to have a couple free days to get ready for them. You'll find yourself pulled out of bed at all hours of the night, spending weekends in the office analyzing data, and you'll spend the holidays hoping that the general public will behave themselves so that nothing burns down (they won't, and it will).
A regular schedule isn't really in the job description, but since you've probably already spent a few years as a firefighter, you're likely to be used to it by the time you graduate to investigator.
You'll also have other stresses that the average firefighter doesn't have to deal with. For instance, their interaction with the public usually involves saving lives and throwing candy from the back of fire trucks during parades (firefighters are awesome). Your interaction with the public usually involves the word interrogation. That's not a word usually known to build strong friendships between people.
Some witnesses will be openly distressed or in shock from the loss of their homes, while other people you interview may have actually set the fire but really don't want to tell you that. Then there's the paperwork, faulty analyses, court dates, testimonies, public hearings, and other civil service minutiae that'll give you blazing headaches.
With all that stress, you say, why not just stay a firefighter? Because the one thing you'll likely never have to deal with is an actual fire. That breakup is a pretty fair stress trade if you ask us.