there is the obvious strategic disadvantage: salonica is too close to the borders and exposed to anyone can control a passage through the central Macedonian plain.
But,
there's also a parameter concerning the power pattern of the Hellenic state: the post-revolution Hellenic state was built on the foundation of specific social groups, which had the political and economic power, and they shared it through a network based on various degrees of relations (from family ties to joint control of electoral districts). This network was founded on the regions of Peloponnesos, Sterea and the Cyclades, and had (actually still has) as centre the capital of the state. The political power power of the members of this network was preserved and expanded, through their involvement in the organization and manning of the various ministries, and as it evolved, their power would seize if they 'd lose contact with the public administration.
It is thus obvious that those people would never accept that the capital moves to another city, especially since many of them were shipowners based on their islands, as Hydra, Poros, Andros, etc.