What are the factors that make g the least at the equator and maximum at the poles ?

Acceleration due to gravity at surface of earth depends on latitude $\left(\phi \right)$ and rotational speed $\left(\omega \right)$ of the earth as,

$g_{\phi }=\mathrm{g}-{\mathrm{R\omega }}^2{\cos }^2\phi$

a) At equator:
$\begin{array}{l}\mathrm{\phi }=0\\ \Rightarrow g_{\phi =0}=\mathrm{g}-{\mathrm{R\omega }}^2{\cos }^{2}0\\ \Rightarrow {\mathrm{g}}_{\mathrm{\phi }}=\mathrm{g}-{\mathrm{R\omega }}^2=\frac{GM}{R^2}-R{\omega }^2\end{array}$

Also, R is maximum at the equator.
$\therefore$g is minimum at equator.
b) At Pole:

$$\begin{gathered} \phi ={90}^o \\ \Rightarrow g_{pole}=g=\frac{GM}{R^2} \end{gathered}$$ 

Also, Radius at the pole is minimum, hence, g is maximum at pole.