The reason is straightforward as David Leonhart notes in the NYTimes:
This is the basic issue. All the young Obama voters did not vote in the midterm elections, so the Republicans won. Ryan rewards his constituency. Taxes will be cut. Spending for the poor will be cut. The deficit, well it will be cut if unemployment magically can fall to 2.8%, and if unicorns can fly.
Yet there is at least one big way in which the plan isn’t daring at all. It asks for a whole lot of sacrifice from everyone under the age of 55 and little from everyone 55 and over. Representative Paul Ryan, the Wisconsin Republican who wrote the plan, calls the budget deficit an “existential threat” to the United States. Then he absolves more than one-third of all adults from responsibility in dealing with that threat...
The reason is partly political. Older people vote in larger numbers than younger adults. Children, of course, can’t vote at all.
Like all good Republican policies, the benefits go to me and the sacrifice goes to the future -- the young who will be left with the cost of the environmental cleanup and a huge federal debt -- and the poor who live elsewhere. I really don't understand why I should be so lucky.