It's pretty much common knowledge that plants planted in pots or soil that doesn't have good drainage can't be watered too much, or they'll "drown". Regardless of what actually happens, experience seems to show that it's true that plants planted in soil can't handle too much water in the soil.
Yet on the other hand, in hydroponics, plants can grow and grow very well indeed with their roots entirely submerged in water. Clearly, it's not about "drowning".
So what's the deal? Why do soil-planted plants die when they're watered too heavily?
(I wasn't sure if this is a Biology.SE or Gardening.SE question, but I figured it was biology since I'm looking for a more theoretical answer about what happens to the roots, rather than practical advice.)