Why is the melting point of magnesium oxide higher than aluminium oxide?
Two ideas to consider:

Alumina is not fully ionic.  Neither is magnesia, but magnesia has more ionic character and we may see the greatest amount of ionic attraction in magnesia versus alumina which is less ionic, or soda which has only singly charged cations.

When metal oxides are melted they do not necessarily produce free metal and oxide ions.  As described by Shi et al. [1], molten alumina retains mostly four-and five-coordination of oxygen to aluminum, rather than forming &quot;free&quot; aluminum and oxide ions and also contrasting with the six-coordination of aluminum in the solid conundrum phase.  Presumably magnesia would behave similarly to alumina when melted, but the greater ionic bonding character in magnesia makes reduced coordination less favorable and thus enhances retaining the fully octahedrally-coordinated solid phase.


Reference
1.
Caijuan Shi, Oliver L. G. Alderman, Diana Berman, Jincheng Du, Joerg Neuefeind, Anthony Tamalonis, J. K. Richard Weber, Jinglin You and Chris J. Benmore,  &quot;The Structure of Amorphous and Deeply Supercooled Liquid Alumina&quot;,
Front. Mater. 6:38 (19 March 2019), https://doi.org/10.3389/fmats.2019.00038.