Because Democrats tend to be poorer and therefore move more often, which requires a change of address form and a new polling place. This whole stupid "voter fraud" thing was devised based on demographics by Republicans to throw Democrats off the voting rolls.
For every case of voter impersonation, about 500 people are disenfranchised because of this. The GOP honchos have talked about this a lot and there are conversations online about the intent of this nonsense. The REPUBLICANS maintain that photo IDs would benefit their candidates.
Here in Miami, nearly all voter fraud involves absentee ballots and local Cubans, who normally vote for Republicans.
If you have 100 million people voting, it is never going to be possible to eliminate all fraud. But what we have is around 99.9% and that is better than disenfranchising 5 to 8% because they moved to a different precinct.