Why are panels of land-rovers made from aluminum?

Not being an automotive engineer, I can't say for certain, but aluminum has some very useful properties:

1. It does not rust beyond the outermost molecular layer, so water will not significantly weaken it.

2. It is very lightweight, important for gas mileage and such, especially with something as big as a Land Rover.

3. Unlike steel, it retains most of its strength even at freezing temperatures. At normal room temperature, steel is (by some measures) over 20 times stronger than aluminum. However, it quickly becomes more brittle as it chills, and will, at 0c, actually break more easily than an equivalent piece of aluminum. The aluminum also weakens as it chills, but much more gradually. Its effective strength at room temperature may not be much, but it's nearly the same at 0c. So you need more aluminum (and/or a stronger configuration) to hold up at room temperature, but you don't have to worry about it breaking in the cold.

(A lesson well illustrated by the WWII-era "Liberty Ships." Steel cargo ships which worked very well until they were brought through the chill temperatures of the northern Atlantic... at which point they snapped in half, one after another.)

4. It is also cheap, plentiful, easily molded, and readily recycled - all potential reasons to choose it over, say, carbon composite (which is also lightweight, strong, and rust-resistant).

5. It's shiny, or can be made so. People like shiny things.

I think it is also to do with availability. When the first Land Rovers were produced, just post WW2, there was lots of recycled aluminium around, from scrapped war planes. They were also, initially, an agricultural vehicle, so designed to have a long working life. This then became tradition at L/R, to have a lot of aluminium panels on their vehicles.