Because it is a secure storage and a secure computation device.
-Physical protection - It is hard to open a smartcard and access data in it. It is not impossible, but is much harder than tampering workstations and PCs.
-Restricted API - A smartcard exports a minimal set of interfaces to avoid exporting flawed interfaces that lead to vulnerabilities.
-Mobility - A user can carry a smartcard, physically separating it from the outside world.
So it is a good place to hide secrets.