Q:

Why did Archelaus move the capitol of Macedon to Pella?

Archelaus transferred the capitol of Macedon from Aegae to Pella in the early 4th century B.C. What could have been the reason for this?

A:

Location.
According to A Companion to Ancient Macedonia edited by Joseph Roisman, Ian Worthington, the "site was attractive for several reasons". 

The location controlled a crossroads,"controlling major passageways
from east to west, and south to north."
The location was also situated on a lake, with a port which may have
been able to be reached from the sea.
The location was also located on a 'fertile hinterland' for food
production.

Concerning Archelaus, we have what Thucydides, History of the Peloponnesian War  said to go on when discussing the invasion of Macedonia from Thrace to the north (emphasis mine):

...retired all within their strongholds and walled towns, as many as
  the country afforded, 4 which were not many then, but were built
  afterwards by Archelaus the son of Perdiccas when he came to the
  kingdom, who then also laid out the highways straight and took order
  both for matter of war, as horses and arms and for other provision,
  better than all the other eight kings that were before him.

So Archelaus is credited with more military growth then all Macedonian kings before him. Specifically for fortification of cities.  As mentioned  above, Pella lies along a main North-South route, along the Varder-Axios river valley. This would be a natural route from the interior lands into Macedonia. Fortifications built here control your northern frontier. This is discussed in In the Shadow of Olympus: The Emergence of Macedon By Eugene N. Borza
The availability of resources, such as grain and fresh water, though it may seem of economic value to us, is also of prime military importance. When Sitalces invaded through that northern frontier, as mentioned by Thucydides , the Macedonian population "retired all within their strongholds and walled towns". If you had the resources you could do that. The Wikipedia article, though not very in-depth, says his invasion ended when "He was obliged to retire through failure of provisions." (A bit of diplomacy and a marriage helped. Emphasis mine)
So the location offered control of the northern frontier, access to the sea, and resources to enable one to be able to withstand a siege. Looks like a good choice to me. Location, Location, Location.

