Prohibiting bank runs

The new rules for money market funds restrict withdrawals under certain conditions.

Yahoo: A large portion of the financial markets, and one that played a pivotal role during the financial crisis, is about to undergo substantial changes.
At the core of the rules is an effort to stabilize the $2.7 trillion industry, which provides a generally safe climate where both retail and institutional investors can park funds and earn modest returns with the bonus that they will never lose money.
For a brief moment in time, that compact was broken.
In 2008, as the financial world crumbled under the weight of the financial crisis that took down Wall Street and threatened to crush the global economy, one fund did the unthinkable: It "broke the buck." That means its share price fell below $1 due to heavy losses it experienced from the collapse of Lehman Brothers. The move by the Reserve Primary Fund set off a wave of panic over possible redemptions and the implosion of a sector that at the time boasted nearly $3.5 trillion in assets.
The new rules seek to prevent a similar occurrence in the future. They allow all funds to block investors — using what are known as "gates" — from withdrawing cash during times of market volatility, a move that coincides with a mandate that retail and government funds maintain a $1 share price. The rules allow prime institutional funds to move to a floating value, meaning they have the possibility both for greater return and greater risk.

Depositors bear the risk and may be forced to leave money in the account much longer.  That means the money market curve is shifting right, it is no longer a demand deposit to park money short term.  So the players are moving to the government debt markets.