This answer assumes that the spacecraft is in an orbit which is mostly aligned to the ecliptic plane.
A target, whose orbit is also close to the ecliptic plane, can occasionally go in front of the Sun or behind it. In both cases, the star tracker camera should not point towards the Sun, even by accident.
It may damage the sensor.
Planets orbit the Sun and have complicated apparent movements. Compared to that, stars can be assumed to be inertially fixed. Computation becomes much easier. No ephemeris needs to be stored.
Canopus's brightness and location well off the ecliptic make it useful for space navigation. Many spacecraft carry a special camera known as a "Canopus Star Tracker" plus a Sun sensor for attitude determination.
Canopus seems to have a significant declination (latitude, not declination; see the other answer) (approximately 52°) which makes it well separated (angular) from the Sun. So it is safer for the tracker camera.
It is the second brightest star in the night sky. Sirius is the brightest.
As pointed out by asdfex, Canopus is farther away from the solar system compared to Sirius. 310 light years versus 8.6 light years. This means that the apparent angular position change (parallax) for different points on the spacecraft orbit will be smaller for Canopus.