The story I was was told relates to osmotic pressure. For the cell to achieve an active metabolic state it has to pull a lot of water into itself to restart its cellular functions. In a solution full of sugar, the sugar tends to pull water into the solution, out of the cells. It is therefore harder for the cell to draw water in. Many cells do not survive the hydration step, and it is assumed (maybe been measured) that more cells die when trying to rehydrate in wort.
Of course, when you pitch a rehydrated cell into wort, the osmotic shock will kill some number of cells. Don't ask me which is the healthiest way for the poor yeast cell.