There's a forum post here that has some interesting information:
Well, there was SOME justification for the decision to cast the
propellant in Utah-- the cool dry climate there, especially the low
humidity, helped considerably in getting a CONSISTENT propellant
casting, especially in casting multiple segments for a solid rocket
Aerojet had created the world's largest solid rocket motor, a 260 inch
behemoth which was static tested as a possible (cheap? or so the
thinking went) replacement for the Saturn IB first stage (cluster's
last stand) instead of the nine clustered tanks, thrust structure, and
eight clustered liquid engines used on that stage. They cast it in
Florida (IIRC) because the 22 foot diameter and massive weight
precluded moving it very far if at all. They test fired it in a 'silo'
test stand (which is there to this day, but abandoned) and there were
some difficulties encountered with the casting of the grains and the
motor itself which were basically 'traced back' to variable
temperature and humidity messing up the pours. The engine didn't
perform as expected, and basically broke the test stand from the
vibration, etc. so they abandoned the idea.
A few years later when shuttle decided to go with solid rocket
boosters, Morton Thiokol, who had extensive experience with solid
rockets from their ballistic missile work for Air Force, won the
contract. Their facilities were in Utah, and really sold the idea that
their products (large ICBM solid rocket motors for missiles like
Minuteman) had given them considerable experience in this area, and
they wouldn't have the problems casting the much-larger shuttle SRB
segments because the dry climate in Utah made casting pours more
consistent and easier. BUT, because of the weight of the fully-fuelled
segments, the only realistic way to transport them was by rail-- they
would be too heavy and too big for road transport, and MUCH too heavy
for air transport, and Utah doesn't have any water shipping ports
capable of shipping them by barge. This meant that the SRB's were
ultimately limited in diameter, which of course in a core-burning
solid rocket, determines the burn duration (more or less).
The Aerojet 260" booster was an attempt to get around size limits of solid rockets. According to the forum post, this attempt failed.
Note that ESA decided to use a segmented booster design for Ariane 5, as well. The top segment is fueled in Italy, the two lower segments are fueled at the Kourou spaceport. They are joined by a construction that is similar to the Shuttle SRBs.