Why are there so few supernovae in M31 (Andromeda)?
The low supernova rate in M31 can be directly attributed to the fact that the galaxy's star formation rate is much lower than the Milky Way's.

Andromeda is currently in a relatively quiet phase in terms of star formation, currently experiencing rates of $\sim0.40M_{\odot}\;\mathrm{yr}^{-1}$. The Milky Way, on the other hand, has a star formation rate several times higher; as such, it contains more massive, short-lived stars which are supernova progenitors. This leads to a correspondingly higher supernova rate than Andromeda. Estimates of the supernova rate in M31 vary, but a value of 0.5 to 1.0 per century is reasonable. This in turn matches well with observations.