Re: It's not "why", it's "how"
A variety of techniques have been alleged in the past to deter or otherwise create a barrier to a citizen voting. That it is blatant gerrymandering based upon an exaggeration of the actual amount of voter fraud is the issue.
There also did use to be a law that meant that States with a history of gerrymandering through voter registration (poll taxes, see Harman v. Forssenius for instance, or literacy tests) had make extra effort to counter such practices. (the Voting Rights Act of 1965)