Why is the dead sea so salty?

The Dead Sea is salty because it has no outlets. Any minerals which flow into it stay there. All bodies of fresh water have outlets which allow them to dispose of dissolved minerals which flow into them from their sources. Oceans and seas typically have no outlets (although there are exceptions; the Mediterranean Sea is saltier than the Atlantic Ocean, and loses mineral-laden water by a deep outflow layer).
There are three factors which contribute to the salinity of the Daed Sea. First, the Dead Sea was formed by a rift in two tectonic plates. The spreading of the two plates created a low spot, which is still sinking, near several rivers. Then, it is land locked, and several rivers lead to the Dead Sea, but no rivers carry water away from the Dead Sea. Finally, the Dead Sea is located in a very hot climate. So, a very sunken body of water is continually being fed more salt water from rivers where the hot temperatures evaporate the water leaving the salt minerals behind.
the dead sea is below sea level and so can't drain into the sea. salt is dissolved into the river Jordan which empties into the dead sea. because it is in a desert there isn't enough water to fill it up and flush it into the sea so the salt accumulates. if there was adequate water fall the region (large parts of Israel and Jordan) would be a massive fresh water lake but the desert doesn't allow this to happen.