Because these provisions disenfranchise qualified and eligible Arizona voters by requiring citizens to present documentary proof of their citizenship status when registering to vote, and further requiring qualified and registered voters to present additional identification at the polling place on Election Day.
[ . . . ]
“Proposition 200 is a barrier that keeps people away from the polls,” said Elliot Mincberg, Legal Director of PFAWF. “We should make it easier for Arizonans to vote, not harder.”
District Court Judge Roslyn Moore-Silver ruled against the plaintiffs in 2008 . A three-judge panel of the Ninth Circuit ruled in 2010  to uphold voter ID but strike down proof of citizenship for voter registration.
"This is a great victory for voting rights advocates," said LULAC General Counsel Luis Vera. "The court ruled that AZ Proposition 200 was superseded by the National Voter Registration Act, which does not allow states to require proof of citizenship to register to vote."
On Tuesday, the full Ninth Circuit ruled  the same:
We uphold Proposition 200’s requirement that voters show identification at the polling place, but conclude that the NVRA supersedes Proposition 200’s registration provision as that provision is applied to applicants using the National Mail Voter Registration Form (the “Federal Form”) to register to vote in federal elections.
Importantly, the full court recognized the additional burden that the proof of citizenship requirement placed on voters, saying :
In short, much of the value of the [NVRA Form] in removing obstacles to the voter registration process is lost under Proposition 200’s registration provision.
As to that point, the state had contended  that “its proof of citizenship requirement [was] not excessively burdensome,” though burdensome nonetheless.
Also known as the Taxpayer Citizen Protection Act, Proposition 200  remains the ALEC  model  for voter ID and proof of citizenship nationwide. Compared side-by-side, you can see that the two measures are virtually identical.
For more information on ALEC in Arizona, check out the November 2011  report and the April 2012  update , co-authored by PFAW Foundation, Common Cause, Center for Media and Democracy, and Progress Now.
For more information on voting rights, check out The Right to Vote under Attack: The Campaign to Keep Millions of Americans from the Ballot Box , a Right Wing Watch: In Focus  report by PFAW Foundation.