The media reports I've seen have made little attempt to explain exactly why Burma's dictators are restricting large numbers of foreign aid workers from entering the country to help with the ongoing disaster. I did volunteer work in eastern Burma in the early 1990s, in an area controlled by the ethnic minority Karen people who rebelled against the dictatorship. The ethnic aspect can help explain how the dictatorship is acting.
The worst-hit area in Burma is the Irrawaddy Delta, which is also has large numbers of the Karen minority. That area had a Karen-led insurrection from the 1940s through the 1960s, with a brief flare-up in the late 1980s. The dictators want no contact between these villagers and the outside world, both for what the villagers could tell the world and for the opportunity given to local people to think they have any rights or expectations.
An additional explanation that I've seen almost nothing about is superstition. There seemed to me to be a fair amount of belief even among educated Burmese that inauspicious events have a semi-supernatural cause. It wouldn't be surprising to see this in Burma about Cyclone Nargis; we saw the same nonsense here in the States over Katrina. A look at the map makes Burma seem like a conscious target of the cyclone, just as the dictatorship is holding a sham referendum to legitimize its rule. I'd expect them to minimize the cyclone's importance, and above all to do nothing to delay the referendum on the basis that it is "inauspicious."
Brutal, superstitious, and ignorant is no way to run a country. Something we've found out recently here at home.