My understanding was that since Florida is essentially at sea level the water table was high enough that it would not be safe or practical to put electrical wires underground. Could be wrong though.
In Weston, FL, which was developed in the 1990 and really took off during the recent bubble the greater part of residential power lines are underground, and consequently they had little trouble after Wilma and Katrina. They are on the western edge of Miami-Palm Beach metro area, where the water table would be higher because of proximity to the Everglades.
I don't think this is an engineering problem. This is a political issue - it would be too difficult to find the money for the conversion, even more so, since in the US taxes are locally raised and spent. Try finding sufficient funds to get the work done in, say, Opa-Locka, or try to raise funds in Palmetto Bay to pay for Homestead.
Sea level goes up in a hurricane, way up. And the only thing worse than freshwater flood for underground wires is saltwater. And if the ground mounted transformers are flooded and destroyed you're losing power. And once power is cut, the power co can't just flip a switch without burning down the whole city, because flood damage means 0.1% of houses will catch fire and the fire dept is already busy with their own issues not to mention trees in the road and such.
Also it may well be that above ground lines and the higher damage/maintenance costs are still cheaper than acquiring easements and all the other costs of installing underground utility infrastructure.
