Its government regulated and the government will want to wet there beak so you have a choice of give them a huge tax windfall or let the government invest it in government backed securites. Either way they get there share.
Here is a quote that might explain it better from the powerball website
"WHY IS THERE A DIFFERENCE BETWEEN THE CASH PAYOUTS IN THE POWERBALL AND MEGA MILLIONS GAMES?
There used to be a larger difference between the two games, but the Mega Millions game was recently changed to mirror the Powerball annuity option. The difference (spread) between the cash and annuity jackpots amounts are now about the same. Both games offer an annuity that pays out in 30 payments over 29 years (the first payment is immediate). Both annuity options are graduated.
The very slight difference now is due to the fact that the Powerball annuity jackpot builds in a 4% per year graduated payment (the next year's payment will be 4% higher the previous year). The Mega Millions game uses a 5% graduated option, which keeps more money invested longer. That gain in earnings is somewhat offset by the different investments used by the two games. Mega Millions is invested in U.S. government strips. Powerball is invested in a couple dozen types of securities; all backed in some way by the U.S. government or agencies. It is important to understand that the lottery is invested in these securities; not the winner. The winner has a contract with the states to pay the annual prize payment, no matter what happens to the securities.
A fixed percentage of every Mega Millions and Powerball ticket sold goes into its CASH jackpot. The cash jackpot is all the money that the lottery has on hand from the sale of tickets in the game. If a player chooses the cash option, then the lottery will pay the entire cash amount to the winner (less income tax withholding amounts required by federal and state laws).
U.S. lotteries offer an ANNUITY jackpot option that can help reduce taxes and offers the winner a 100% guaranteed income stream over time. If a winner elects the annuity option, then the lottery will invest the entire cash amount BEFORE taxes are deducted. The difference between the CASH jackpot and the ANNUITY jackpot is the interest earnings that build over time."
http://www.powerball.com/pb_contact.asp