Various reasons, all curiously surrounding the trust issue. To use an exchange, you need to trust it. I trust Kraken because

 - It's run by Jesse Powell, with a solid and proven track record.
 - Kraken has a 100% solid track record.
 - They do not engage in super crazy volume hunt.
 - US/German based, with solid banking partners.
 - They were chosen to handle the Mt.Gox claim case, so that means the US/Japanese government did it's homework and chose Kraken to handle the cryptos.
 - They are constantly in the front line of regulatory acceptance, which speaks volumes for the quality of their CxO's
 - You can at any time audit your account to verify if you do actually have your funds as displayed.

 Some other exchanges have merits too, and I'm only talking about the "serious" of them, Finex, Stamp, Polo etc.

 To this day, the only major exchange that has never been hacked is Kraken.

 So in a trustless environment where you still need trust to perform price discovery, Kraken is #1 in my top 3 trust billboard. Probably last in GUI though.